Increasing aridity, temperature and soil pH induce soil C-N-P imbalance in grasslands.

Due to the different degrees of controls exerted by biological and geochemical processes, climate changes are suggested to uncouple biogeochemical C, N and P cycles, influencing biomass accumulation, decomposition and storage in terrestrial ecosystems. However, the possible extent of such disruption in grassland ecosystems remains unclear, especially in China's steppes which have undergone rapid climate changes with increasing drought and warming predicted moving forward in these dryland ecosystems. Here, we assess how soil C-N-P stoichiometry is affected by climatic change along a 3500-km temperate climate transect in Inner Mongolia, China. Our results reveal that the soil from more arid and warmer sites are associated with lower soil organic C, total N and P. The ratios of both soil C:P and N:P decrease, but soil C:N increases with increasing aridity and temperature, indicating the predicted decreases in precipitation and warming for most of the temperate grassland region could lead to a soil C-N-P decoupling that may reduce plant growth and production in arid ecosystems. Soil pH, mainly reflecting long-term climate change in our sites, also contributes to the changing soil C-N-P stoichiometry, indicating the collective influences of climate and soil type on the shape of soil C-N-P balance.

Aridity has increased over most land areas since 195012 and this widespread drying trend has been enhanced by recent global warming3. In this century, global drought areas could double due to the decreased precipitation and increased evaporation, exacerbating processes that lead to land degradation and desertification245. Meantime, global warming will continue for centuries to come, likely increasing the risk of both floods and droughts3. In particular, significant warming that occurs mostly in arid and semi- arid regions will make drought worse in these regions6. Given the importance of n class="Chemical">water availability aclass="Chemical">nd temperature oclass="Chemical">n biological aclass="Chemical">nd geochemical processes, the predicted dryer aclass="Chemical">nd warmer climate will greatly alter biomass accumulatioclass="Chemical">n, decompositioclass="Chemical">n aclass="Chemical">nd storage iclass="Chemical">n differeclass="Chemical">nt ways, aclass="Chemical">nd likely disrupt the biogeochemical cycles of C, N aclass="Chemical">nd P7, class="Chemical">negatively affecticlass="Chemical">ng the provisioclass="Chemical">n of key services provided by these ecosystems8910.

Concentrations of elements such as C, N and P in soils do not necessarily change in unison with increasing aridity and temperature, possibly leading to elemental imbalance that negatively affects plant growth and production. In spite of numerous studies on the responses of soils and plants to drought and warming, their effects on elemental balance have received very little attention. Microelements of C, N and P are vital for ecosystem processes, but little is known about their response to drought and warming. With increasing drought and temperature, the biological processes that drive C and N inputs and fluxes in ecosystems may be impaired due to moisture limitation at warmer conditions, resulting in soil C and N decline. In contrast, the availability of soil P is less affected by biological processes but more affected by mechanical rock weathering. The different direction and magnitude of C, N and P in response to drought in warm environment could result in a n class="Chemical">C-N-P imbalaclass="Chemical">nce. However, the effects of iclass="Chemical">ncreasiclass="Chemical">ng drought aclass="Chemical">nd temperature oclass="Chemical">n C:N:P ratios iclass="Chemical">n terrestrial ecosystems remaiclass="Chemical">n uclass="Chemical">nclear. Iclass="Chemical">ndeed, class="Chemical">no studies have measured soil C:N:P ratios iclass="Chemical">n respoclass="Chemical">nse to experimeclass="Chemical">ntal drought or warmiclass="Chemical">ng treatmeclass="Chemical">nts iclass="Chemical">n fields. There are a few studies oclass="Chemical">n placlass="Chemical">nt C:N:P ratios but the reported results are iclass="Chemical">ncoclass="Chemical">nclusive for the ratios’ respoclass="Chemical">nse to drought aclass="Chemical">nd warmiclass="Chemical">ng11.

Ecological stoichiometry, the study of balance of energy and multiple chemical elements in biological systems ranging from molecules to ecosystems, is a fundamental concept in ecology. It provides a powerful tool for us to advance our understanding of biological processes and nutrient cycling in terrestrial ecosystems1213. In natural habitats, an organism’s C:N:P stoichiometry can be affected by both biotic and abiotic factors, including the predicted global changes such as increasing atmospheric n class="Chemical">CO2 aclass="Chemical">nd climate chaclass="Chemical">nge711. Global chaclass="Chemical">nge-iclass="Chemical">nduced variatioclass="Chemical">ns iclass="Chemical">n C:N:P stoichiometry might alter ecosystem structure aclass="Chemical">nd fuclass="Chemical">nctioclass="Chemical">n by affecticlass="Chemical">ng producer-coclass="Chemical">nsumer relatioclass="Chemical">nships aclass="Chemical">nd/or competitioclass="Chemical">n amoclass="Chemical">ng species11.

Drylands (arid, semi-arid and dry sub-humid ecosystems), covering about 41% of the Earth’s land surface and hosting over 38% of the global population, have always experienced drought. Despite much attention paid to the climate-induced decoupling of biogeochemical cycles and nutrient imbalance in soils and plants8141516171819, our understanding remains limited on how class="Chemical">water shortage affects soil class="Chemical">n class="Chemical">C-N-P balance. This is especially true in China’s grasslands; they cover more than 40% of the total land area of the country and these temperate grasslands in particular have experienced increased drought and warming over 30 years20. From east to west in northern China’s grasslands, the precipitation decreases but the temperature increases gradually and vegetation changes from meadow steppe, typical steppe to desert steppe as aridity increases. Over the long term it is climate that shapes the physical landscape and determines where various ecosystems can exist. Along the precipitation/temperature gradient, vegetation and soil vary in response to the long-term effects of climate. The processes that affect soil C, N and P availability are also expected to change accordingly, but in different ways, thus resulting in changing C:N:P ratios. The grassland’s services such as providing production of food, wood and biofuels, and to offset the emission of greenhouse gasses2122, might be greatly negatively affected by this changing C-N-P balance. Despite the importance of grasslands, it is still largely unknown how the predicted increased drought and warmer conditions will affect soil C-N-P balance. In this study, we evaluated how increasing aridity and temperature influences soil C-N-P balance through analysis of collected soil samples from 65 grassland sites along a precipitation gradient from meadow steppes, to typical steppes, and to desert steppes in northern China. Aside from climate, we also determined how soil C-N-P balance varied with soil types and plant vegetation, both of which mostly reflect the long-term effect of climate change.

Because C and N availability is primarily linked to biological processes but available P is derived mainly from mechanical rock weathering and, to a lesser extent, from the organic matter decomposition23, we hypothesized that increasing aridity, especially at warm conditions could induce moisture limitation and lead to a decline of soil C and N. In contrast, soil P availability was hypothesized to be less affected by climate. Due to the long-term influence of climate on soils, we also hypothesized that soil C, N and P were associated with soil taxa and acidity/alkalinity.

Results

Along this precipitation and temperature gradient, soil organic C, total N, total P, pH, available N and P varied in 2013 at 0.06 ~ 5.87%, 0.01 ~ 0.52%, 0.13 ~ 0.94%, 6.1 ~ 8.5, 2.5 ~ 23.4 mg N kg−1, and 0.9 ~ 9.1 mg P kg−1, respectively, in the soil profile to 30 cm depth (Table 1). Plotting soil nutrients against aridity index revealed significant relationships (Supplementary materials, Figs S1-S4). Soil organic C, total N, available N and total P all decreased with aridity from dry sub-arid, to semi-arid, to arid sites. In contrast, soil pH increased with aridity. Soil available P was not significantly affected by aridity. With increasing temperature from cool to warm sites, soil organic C, total N, available N and total P also decreased. In contrast, soil pH and available P increased with temperature. Reflecting the long-term effect of climate change, soil pH increased with increasing aridity and temperature, tended to have negative effects on soil organic C, total N, available N and total P. The variables of soil C, N and P were closely associated with each other and also associated with the latitude and longitude coordinates of the studied sites (Supplementary materials, Figs S5-S16).

Table 1

The soil characteristics of 65 grassland sampling sites.

Soil type	AI	AB	pH	SOC	TN	TP	AN	AP	TC:N	TC:P	TN:P	AN:P
Anthrosol	0.20	194 ± 22b	8.0 ± 0.1a	0.1 ± 0.02c	0.01 ± 0.00d	0.2 ± 0.01b	5.2 ± 0.3ab	3.8 ± 0.3ab	10.0 ± 2.0a	7 ± 2c	0.7 ± 0.04c	1.4 ± 0.1abc
Arenosol	0.31	201 ± 39b	7.6 ± 0.1ab	1.3 ± 0.3abc	0.11 ± 0.02abcd	0.3 ± 0.03b	7.6 ± 0.6ab	4.0 ± 0.4ab	21.2 ± 2.8a	35 ± 5bc	2.9 ± 0.5bc	3.1 ± 0.5abc
Calcisol	0.25	169 ± 36c	7.9 ± 0.1a	0.7 ± 0.1c	0.06 ± 0.01cd	0.3 ± 0.02b	6.1 ± 0.3ab	4.1 ± 0.3ab	21.1 ± 6.7a	27 ± 4bc	2.2 ± 0.3bc	1.9 ± 0.3abc
Cambisol	0.42	149 ± 35c	6.4 ± 0.1c	2.5 ± 0.3ab	0.26 ± 0.02ab	0.4 ± 0.02b	12.4 ± 2.0a	1.6 ± 0.2b	9.7 ± 0.6a	66 ± 6ab	6.9 ± 0.4ab	7.9 ± 0.9a
Chernozem	0.48	269 ± 11bc	6.5 ± 0.1bc	2.5 ± 0.3ab	0.23 ± 0.03abc	0.3 ± 0.01b	11.6 ± 1.1ab	2.8 ± 0.9ab	10.7 ± 0.1a	95 ± 11a	8.9 ± 1.1a	5.6 ± 2.2ab
Gleysol	0.17	20 ± 3d	8.5 ± 0.1a	0.4 ± 0.03c	0.02 ± 0.00d	0.2 ± 0.04b	6.0 ± 0.4ab	3.4 ± 0.6ab	17.4 ± 0.8a	18 ± 3c	1.0 ± 0.2c	1.9 ± 0.2abc
Greyzem	0.42	325 ± 19bc	7.6 ± 0.02ab	1.4 ± 0.1abc	0.11 ± 0.01abcd	0.7 ± 0.13a	2.5 ± 0.1b	2.8 ± 0.3ab	12.9 ± 0.3a	21 ± 7bc	1.6 ± 0.5c	0.9 ± 0.1bc
Kastanozem	0.33	253 ± 27c	7.5 ± 0.1abc	1.2 ± 0.1abc	0.11 ± 0.01abcd	0.3 ± 0.01b	8.7 ± 0.6ab	3.5 ± 0.2ab	12.3 ± 0.3a	44 ± 2bc	3.8 ± 0.2bc	4.4 ± 0.6abc
Leptosol	0.37	280 ± 21bc	7.7 ± 0.1a	1.2 ± 0.03abc	0.10 ± 0.00bcd	0.4 ± 0.01b	3.0 ± 0.4b	5.1 ± 0.2ab	12.3 ± 0.5a	33 ± 1bc	2.7 ± 0.1bc	0.6 ± 0.1c
Phaeozem	0.48	296 ± 61bc	6.6 ± 0.1bc	2.9 ± 0.2a	0.28 ± 0.02a	0.5 ± 0.05ab	9.2 ± 0.6ab	4.3 ± 0.4ab	10.2 ± 0.3a	67 ± 4ab	6.7 ± 0.5ab	3.1 ± 0.5abc
Solonchack	0.44	462 ± 33a	6.6 ± 0.02bc	2.0 ± 0.1abc	0.18 ± 0.01abcd	0.5 ± 0.01ab	12.9 ± 0.7a	5.8 ± 1.2a	11.2 ± 0.3a	45 ± 1bc	4.0 ± 0.1bc	2.4 ± 0.4abc
mean	0.35	241 ± 19	7.4 ± 0.1	1.5 ± 0.1	0.14 ± 0.01	0.4 ± 0.01	8.2 ± 0.3	3.8 ± 0.1	14.5 ± 1.0a	44 ± 2.0	4.0 ± 0.2	3.5 ± 0.3

Data are means ± 1 SE. The soils are classified according to the Harmonised World Soil Database (HWSD, version 1.21) released by FAO. Different letters indicate significant (P < 0.05) differences among soil types. Abbreviations: AI, aridity index (unitless); AB, aboveground biomass (g m−2); SOC, soil organic carbon (%); TN, soil total nitrogen (%); TP, soil total phosphorus (%); AN, available nitrogen (ppm); AP, available phosphorus (ppm); TC:N, ratio of total carbon to nitrogen; TC:P, ratio of total carbon to phosphorus; TN:P, ratio of total nitrogen to phosphorus; AN:P, ratio of available nitrogen to phosphorus.

For the soil n class="Chemical">C-N-P balaclass="Chemical">nce, the ratios of soil C:N, C:P aclass="Chemical">nd N:P based oclass="Chemical">n soil total N aclass="Chemical">nd P coclass="Chemical">nceclass="Chemical">ntratioclass="Chemical">ns aloclass="Chemical">ng this aridity aclass="Chemical">nd temperature gradieclass="Chemical">nt, varied betweeclass="Chemical">n 7 ~ 67, 5 ~ 118 aclass="Chemical">nd 0.2 ~ 11 respectively. Soil available N:P ratio, calculated from available N aclass="Chemical">nd P, varied from 0.5 to 17.7. We fouclass="Chemical">nd soil C:N iclass="Chemical">ncreased, but C:P decreased quadratically with aridity. Negative quadratic relatioclass="Chemical">ns were observed betweeclass="Chemical">n aridity aclass="Chemical">nd soil total N:P ratios. A such class="Chemical">negative quadratic relatioclass="Chemical">nship was also sigclass="Chemical">nificaclass="Chemical">nt wheclass="Chemical">n available N aclass="Chemical">nd P were coclass="Chemical">nsidered iclass="Chemical">nstead of total N aclass="Chemical">nd P (Fig. 1). Soil C:P, total aclass="Chemical">nd available N:P decliclass="Chemical">ned while soil C:N iclass="Chemical">ncreased with temperature (Fig. 2). Raiclass="Chemical">nfall revealed similar effect oclass="Chemical">n soil C:N:P as aridity aloclass="Chemical">ng the studied sites (Fig. 3). Soil C:N:P was closely related to soil pH values which positively affected soil C:N but class="Chemical">negatively affected soil C:P aclass="Chemical">nd N:P ratios (Fig. 4).

Figure 1

Relationships between aridity index and the ratios of soil C, N and P.

Aridity index is the ratio of precipitation to potential evapotranspiration. The solid dark grey lines represent the fitted quadratic regressions. The ratio values are log-transformed. R2, proportion of variance explained. Generalized climate classification scheme for aridity values on the top is based on UNEP51.

Figure 2

Relationships between mean temperature from May to August, 2013 and the ratios of soil C, N and P.

The solid dark grey lines represent the fitted quadratic regressions. The ratio values are log-transformed. R2, proportion of variance explained.

Figure 3

Relationships between total rainfall from May to August, 2013 and the ratios of soil C, N and P.

The solid dark grey lines represent the fitted quadratic regressions. The ratio values are log-transformed. R2, proportion of variance explained.

Figure 4

Relationships between soil pH values and the ratios of soil C, N and P.

The solid dark grey lines represent the fitted quadratic regressions. The ratio values are log-transformed. R2, proportion of variance explained.

Climate data (temperature and precipitation) collectively explained 34 ~ 77% of the variation in soil organic C, total N and P and their stoichiometric ratios (Table 2). The soil type was also a strong driver over soil C:N:P ratios. Soil pH accounted for between 15 ~ 64% of the influence on soil C, N, P and their stoichiometric ratios. However, only a small fraction of soil C:N:P ratios had associated with the plant cover. When the data of soil types and pH values were also taken into account, the climate variables accounted for a partial 32 ~ 80% of the variation in soil C, N, P and their stoichiometric ratios. Climatic data (temperature and precipitation) combined with soil data (types and pH) and plant cover explained surprisingly high proportions of soil C:N:P variation: 81, 83 and 42% of the variation in soil C N, P concentrations, and 46, 67 and 67% of the variation soil C:N, C:P and N:P, respectively (Table 2). Climatic variables and soil pH generally had the major contribution to the variations in soil C:N:P ratios (Fig. 5).

Table 2

r2 values in multiple regression analyses of soil carbon, nitrogen, phosphorus and their ratios.

Model	SOC	TN	TP	AN	AP	TC:N	TC:P	TN:P	AN:P
Aridity (A)	0.567***	0.520***	0.331***	0.184***	0.001 ns	0.194***	0.324***	0.326***	0.100**
Temperature (T)	0.761***	0.742***	0.306***	0.161***	0.040 ns	0.353***	0.555***	0.568***	0.133**
Rainfall (R)	0.419***	0.370***	0.275***	0.136**	0.008 ns	0.116**	0.224***	0.212***	0.063*
Soil type (ST)	0.336***	0.282**	0.092 ns	0.073 ns	0.034 ns	0.124 ns	0.270**	0.218**	0.003 ns
Soil pH (pH)	0.620***	0.642***	0.154***	0.308***	0.098*	0.372***	0.542***	0.567***	0.268***
Plant cover (PC)	0.088*	0.079*	0.073*	0.005 ns	0.016 ns	0.020 ns	0.030 ns	0.033 ns	0.009 ns
T + R	0.765***	0.747***	0.336***	0.161**	0.041 ns	0.359***	0.558***	0.576***	0.119**
T + R + ST	0.779***	0.785***	0.419***	0.244***	0.010 ns	0.371***	0.592***	0.572***	0.125*
T + R + ST + pH	0.802***	0.827***	0.426***	0.321***	0.112 ns	0.460***	0.663***	0.665***	0.268**
Overall	0.810***	0.831**	0.427***	0.289**	0.097 ns	0.458***	0.670***	0.667***	0.239*
stepAIC	−213	−181	−225	−205	−189	−260	−221	−177	−136
TotalAIC	−211	−170	−214	−200	−180	−252	−210	−170	−126

ns (not significant, P > 0.05), *(P < 0.05), **(P < 0.01), ***(P < 0.001). The ‘+’ includes the relevant predictors and their interactions. The models labelled ‘Overall’ includes temperature, rainfall, soil pH and plant cover as explanatory variables. StepAIC: AIC value of model selected by the stepwise procedure. TotalAIC: AIC value of model with all parameters. AIC is the Akaike Information Criterion.

Figure 5

The relative influence of climatic and soil variables and plant cover on soil C, N and P.

The relative importance values add to 100% for each model. In each model, predictors with large relative importance values have greater explanatory powers. See abbreviations in Table 1.

Discussion

Our findings suggest that aridity, temperature and soil acidity/alkalinity are important drivers of soil stoichiometric C, N and P in our studied sites. Because natural soil acidity/alkalinity mostly reflects long-term change of climate, our results highlighted the abiotic controls over the soil n class="Chemical">C-N-P balaclass="Chemical">nce iclass="Chemical">n drylaclass="Chemical">nds. Giveclass="Chemical">n that drylaclass="Chemical">nds are more N- aclass="Chemical">nd P-limited thaclass="Chemical">n other ecosystems24, the decreases of soil C, N aclass="Chemical">nd P with aridity aclass="Chemical">nd temperature revealed iclass="Chemical">n our aclass="Chemical">nalyses imply that the iclass="Chemical">ncreasiclass="Chemical">ngly severe droughts aclass="Chemical">nd warmiclass="Chemical">ng iclass="Chemical">n drylaclass="Chemical">nds2 might lead to more severe N aclass="Chemical">nd P depletioclass="Chemical">n iclass="Chemical">n arid aclass="Chemical">nd warm regioclass="Chemical">ns, especially iclass="Chemical">n the most arid aclass="Chemical">nd warm sites of the west eclass="Chemical">nd of class="Chemical">northerclass="Chemical">n Chiclass="Chemical">na’s temperate grasslaclass="Chemical">nds.

In our study, we found that soil C, N and P all decreased with increasing aridity and warming. No data for soil C, N and P in response to drought or warming treatments are available so far for comparison. However, studies on plants reveal that drought treatments could enhance the concentrations of C and N but reduced the concentration of P in the bryophyte class="Species">Hypnum cupressiforme Hedw. growiclass="Chemical">ng iclass="Chemical">n a Mediterraclass="Chemical">neaclass="Chemical">n forest25. Because placlass="Chemical">nt class="Chemical">nutrieclass="Chemical">nts may class="Chemical">not always reflect the class="Chemical">nutrieclass="Chemical">nt availability iclass="Chemical">n soils, how soil C, N aclass="Chemical">nd P respoclass="Chemical">nse to drought aclass="Chemical">nd warmiclass="Chemical">ng treatmeclass="Chemical">nt iclass="Chemical">n Mediterraclass="Chemical">neaclass="Chemical">n forests aclass="Chemical">nd whether the respoclass="Chemical">nses are similar to our results remaiclass="Chemical">ns uclass="Chemical">nclear. The aridity- aclass="Chemical">nd warmiclass="Chemical">ng-related decliclass="Chemical">ne of soil N has beeclass="Chemical">n coclass="Chemical">nfirmed by class="Chemical">natural abuclass="Chemical">ndaclass="Chemical">nce of class="Chemical">n class="Disease">N isotopes (δ15N) along a precipitation/temperature gradient in Inner Mongolian grasslands26. However, adding water treatments in the same ecosystem reduced soil C and N but did not affect soil P27. Experimental warming also revealed that increasing soil temperature could enhance soil N and P availability in grasslands28293031, indicating differing responses of soil nutrients to drought/warming between short-term manipulative experiments and long-term gradient observations.

The increase of soil C:N and decrease of soil N:P with aridity in our study were similar to the drought effects on a bryophyte in Mediterranean forest25. The C:N and C:P ratios of the leaves of Mediterranean shrubs and trees were also enhanced by drought32. Similar to our results, soil N:P ratios were found to be greater in meadow steppe than in desert steppe33 in Inner Mongolia. However, no effect of drought on bryophyte C:P ratios in Mediterranean forest was also found25. Similarly, the C:N ratios in the roots of n class="Species">Quercus ilex, a deciduous tree species, were reduced by drought treatmeclass="Chemical">nt iclass="Chemical">n semi-arid areas iclass="Chemical">n the Mediterraclass="Chemical">neaclass="Chemical">n Basiclass="Chemical">n34. Drought has beeclass="Chemical">n fouclass="Chemical">nd to iclass="Chemical">ncrease the C:N ratio iclass="Chemical">n temperate heathlaclass="Chemical">nds35 but decrease the C:N ratio iclass="Chemical">n wet-temperate ecosystems11, suggesticlass="Chemical">ng that the respoclass="Chemical">nses of stoichiometric ratio to drought may differ amoclass="Chemical">ng ecosystem types aclass="Chemical">nd species-specific.

Similar to our soil results, experimental warming was found to enhance C:N and C:P but reduce N:P ratios in plant tissues363738. However, stoichiometric N:P in plant tissues was also found to decline in warming experiments39. A global meta-analysis of manipulative experiments revealed that experimental warming tends to reduce plant C:N and enhance C:P and N:P7, again suggesting differences of responses between soils and plants, and differences between long-term and short-term responses.

Decreasing soil C and N with aridity can be attributed to the aridity-induced decline of soil n class="Chemical">water availability aclass="Chemical">nd vegetatioclass="Chemical">n cover, both of which directly or iclass="Chemical">ndirectly affect C- aclass="Chemical">nd N-related processes, particularly biological processes such as photosyclass="Chemical">nthesis, atmospheric N fixatioclass="Chemical">n, aclass="Chemical">nd the activity of microbes aclass="Chemical">nd soil eclass="Chemical">nzymes404142. The iclass="Chemical">ncreasiclass="Chemical">ng temperature with aridity might worseclass="Chemical">n the drought effects oclass="Chemical">n soil C, N aclass="Chemical">nd P availability. Iclass="Chemical">n additioclass="Chemical">n, reduced vegetatioclass="Chemical">n cover, together with iclass="Chemical">ncreased temperature, eclass="Chemical">nhaclass="Chemical">nces soil dryiclass="Chemical">ng, promoticlass="Chemical">ng soil erosioclass="Chemical">n that caclass="Chemical">n remove ficlass="Chemical">ne, class="Chemical">nutrieclass="Chemical">nt-rich particles such as clay40. These processes caclass="Chemical">n result iclass="Chemical">n aclass="Chemical">n aridity-related soil N decliclass="Chemical">ne although low raiclass="Chemical">nfall aclass="Chemical">nd high evaporatioclass="Chemical">n iclass="Chemical">n arid sites may preveclass="Chemical">nt available class="Chemical">nutrieclass="Chemical">nts from beiclass="Chemical">ng washed out of the soil profile. Giveclass="Chemical">n the stroclass="Chemical">ng photosyclass="Chemical">nthesis vs. N relatioclass="Chemical">nship434445, the aridity-iclass="Chemical">nduced reductioclass="Chemical">ns iclass="Chemical">n N availability will limit placlass="Chemical">nt productioclass="Chemical">n capacity that could have mitigated the rise of atmospheric class="Chemical">n class="Chemical">CO2 increase in a negative feedback, thus leading to a warmer world.

Similar to the soil total N pattern, soil total P along our 3500-km transect sites also decreased with aridity and warming. However, soil available P was not affected by aridity. With increasing aridity, biological weathering, i.e., the organic matter decomposition, decreases46, but mechanical rock weathering may increase4247. The non-significance of the overall effect of aridity on class="Chemical">inorganic P iclass="Chemical">n soils aloclass="Chemical">ng our studied sites iclass="Chemical">ndicates that the two P-related processes offset partly each other. The decliclass="Chemical">ne of total P with aridity without iclass="Chemical">ncreases iclass="Chemical">n soil available P suggests iclass="Chemical">ncreasiclass="Chemical">ng rock weathericlass="Chemical">ng, which is a physical process aclass="Chemical">nd provides more available P thaclass="Chemical">n orgaclass="Chemical">nic matter decompositioclass="Chemical">n, is the domiclass="Chemical">naclass="Chemical">nt driver of the class="Chemical">noclass="Chemical">n-chaclass="Chemical">nge iclass="Chemical">n soil P availability. Iclass="Chemical">n additioclass="Chemical">n, reduced class="Chemical">n class="Disease">P loss through leaching due to low rainfall and high evaporation may be another driver of changes in P availability in arid sites. Our findings suggest that the levels of total P are expected to increase as soils become drier and erode more. Along our 3500-km transect from meadow steppe to desert steppe, both plant above- and belowground biomass declines with aridity and warming, suggesting that N and P sequestrated by plant biomass also decreases with aridity and warming, thus promoting high available N and P pools in soils, but in differing extent (supplementary materials, Figs S1 and S2). The different trends of soil N and P availability with aridity and warming suggest that the decline of plant production in our arid and warm sites is attributed to the changes in soil N rather than P availability and therefore N is likely the primary limiting nutrients in arid and warm systems.

The trend of aridity-related soil P is different from that by Delgado-Baquerizo, et al.8 who reported that total P was not affected, but class="Chemical">inorganic P was positively affected by aridity across all coclass="Chemical">nticlass="Chemical">neclass="Chemical">nts except Aclass="Chemical">ntarctica. The respoclass="Chemical">nses of soil class="Chemical">nutrieclass="Chemical">nts, particularly P, to aridity at a world scale, therefore, caclass="Chemical">nclass="Chemical">not be applied to regioclass="Chemical">nal scale. The differeclass="Chemical">nce iclass="Chemical">n class="Chemical">n class="Chemical">inorganic P can be attributed to the different pattern of its parent total P which significantly declines with aridity in our study, but does not change in the study by Delgado-Baquerizo, et al.8 at a larger geographic scale. Soil nutrients indeed reflect the overall effect of aridity in the balance of gains (weathering, atmospheric deposition, N fixation and organic matter decomposition) and losses (erosion, leaching and gaseous losses), whether biological or geochemical.

Our analyses reveal that soil class="Chemical">C-N-P balaclass="Chemical">nce is stroclass="Chemical">ngly affected by temperature, apart from raiclass="Chemical">nfall. Soil C, N aclass="Chemical">nd P decrease with decreasiclass="Chemical">ng precipitatioclass="Chemical">n but iclass="Chemical">ncreasiclass="Chemical">ng temperature, suggesticlass="Chemical">ng that global warmiclass="Chemical">ng will exacerbate the drought-iclass="Chemical">nduced class="Chemical">n class="Chemical">C-N-P shortage and imbalance due to the co-variation of temperature and precipitation. Our results also demonstrate that soil properties, especially soil pH values, are strong drivers of soil C, N and P concentrations, indicating the underlying influence of soil pH-related processes on soil C-N-P stoichiometry. Indeed, soil pH values are measures of soil acidity/alkalinity which mainly reflects long-term effect of climate. The close relationship between soil C-N-P stoichiometry and soil pH values was indeed a direct reflection of the climatic effect.

In the less arid sites such as semi-arid and dry sub-humid ecosystems, there are more C and N available for plants and microorganisms to uptake. Simultaneously, available P kept at a relatively high level can couple C, N and P to biological processes. In comparison, soil organic C, total N and P all decline with aridity, but in different degrees, resulting in low C:P and N:P ratios, but high C:N ratio at arid sites. Likewise, N:P ratio class="Disease">declines when inorganic N aclass="Chemical">nd P are coclass="Chemical">nsidered iclass="Chemical">nstead of total N aclass="Chemical">nd P. Similar patterclass="Chemical">ns were fouclass="Chemical">nd for soil class="Chemical">n class="Chemical">C-N-P stoichiometry in relation to temperature. The observed stoichiometric changes suggest that, in response to increasing aridity and warming, plant growth is limited by the decoupling of soil biogeochemistry: C and N become uncoupled from P which is no longer reliant on C and N levels. Soil N also becomes uncoupled from C in arid and warm conditions, implying an imbalance of soil C-N-P concentrations that could constrain plant and microbial activity and thus biomass production and decomposition. Our observations here accord with the results of short-term drought-induced decoupling C-N-P under rainfall manipulation4849.

Although soil organic C, total N and P are strongly associated with each other in our studied sites (Supplementary materials, Fig. S16), the observed aridity- and temperature-related changes in stoichiometric class="Chemical">C-N-P ratios iclass="Chemical">ndicate that future climatic chaclass="Chemical">nges, particularly iclass="Chemical">ncreasiclass="Chemical">ng drought aclass="Chemical">nd global warmiclass="Chemical">ng, will uclass="Chemical">nbalaclass="Chemical">nce soil class="Chemical">n class="Chemical">C-N-P concentrations and therefore likely uncouple their biogeochemical cycles at least in drylands of northern China. The low growth rate of plants and low primary production in arid ecosystems reflects a fragile coupling between these biogeochemical cycles, especially under rapid climate change in these regions. In contrast, dry sub-humid regions (aridity index >0.65) might be more resistant to drought and these ecosystems will probably more easily recover to previous ecosystem stoichiometry post-drought. It should be noted that C, N and P cycling in arid systems can still be coupled (despite being fragile), even when their ratios change, as long as cycling of these elements is controlled by biological processes. Changes in C:N:P stoichiometry do not necessarily imply less control by biological processes. The diminishing interaction between plants and soil life with increasing drought and temperature in drylands of northern China, however, could upset soil C-N-P balance and reduce the resilience of arid ecosystems in a changing world, pushing currently arid regions into full desert aridity.

Conclusion

Our 3500-km transect study in 65 dryland sites of northern China’s steppe, to our knowledge, was the first to explore how aridity and warming influence soil class="Chemical">C-N-P balaclass="Chemical">nce iclass="Chemical">n meadow steppe, typical steppe aclass="Chemical">nd desert steppe, all of which are expected to experieclass="Chemical">nce drier aclass="Chemical">nd warmer climates12. Our results reveal that the predicted iclass="Chemical">ncreasiclass="Chemical">ng drought aclass="Chemical">nd warmiclass="Chemical">ng iclass="Chemical">n drylaclass="Chemical">nds caclass="Chemical">n lower soil C aclass="Chemical">nd N coclass="Chemical">nceclass="Chemical">ntratioclass="Chemical">ns that are primarily liclass="Chemical">nked to biological processes. However, soil P availability is class="Chemical">not affected by aridity although soil total class="Chemical">n class="Disease">P declines with aridity, suggesting that increasing mechanical rock weathering might have exceeded decreasing biological weathering (decomposition of organic matter) with aridity. The different trends of soil total and available P with temperature strengthened this idea. The changes in soil C, N and P across the rainfall and temperature gradient could be attributed to aridity/temperature variations and the rainfall/temperature-induced change in soils and vegetation (e.g. soil pH and vegetation cover), whether through direct or indirect effects. Reflecting by stoichiometric ratio changes, soil C-N-P concentrations are unbalanced and plant production declines accordingly. Therfore, C-N-P cycles might be interrupted in more arid and warm ecosystems, likely negatively affecting biogeochemical-controlled ecosystem functions despite plants have the adaptive capacity. The similar responses of soil C-N-P to declined rainfall and increased temperature suggest that the disturbance of soil nutrient balance in dryland ecosystems could worsen due to the global warming-induced increase in drought.

Methods

Experiment design

Sixty-five grassland sites located in Inner Mongolia, China, were selected for this study. The sites were chosen along a 3500-km-long transect, located in arid, semi-arid and dry sub-humid areas covering a wide spectrum which differs in climates, soil types, vegetation cover and species richness. Mean annual temperature was 2.1 °C, with a tendency for higher temperatures in the south-west. Mean annual precipitation decreased from 457 mm yr−1 to 154 mm yr−1, reflecting the transition from a humid to an arid climate. The corresponding summer rainfall (May to September) in plant growth season decreased from 404 to 133 mm yr−1. Due to the close relation among aridity, precipitation, and temperature, this aridity gradient can also be considered as a precipitation gradient or a temperature gradient.

Soil sample collection and measurement

The spatial geographical coordinates (longitude and latitude) and altitude of each site were obtained by GPS. We established a 30 m × 30 m plot representative of the dominant vegetation at each site and collected 15 soil samples at the 0–30 cm layer in the plot of each site in 2013 with a soil core (5 cm in diameter). Soil samples were taken to the laboratory, air-dried and sieved for laboratory analyses of C, N, P and pH. Soil organic C was analyzed by colorimetry following dichromate oxidation when boiling with a mixture of class="Chemical">potassium dichromate aclass="Chemical">nd class="Chemical">n class="Chemical">sulphuric acid. Total N was measured using the Kjeldahl acid-digestion method. Total P was determined by the sulfuric acid hydrolysis procedure. Available soil N, determined as the sum of ammonium, nitrate and dissolved organic nitrogen, was analyzed colorimetrically using a continuous-flow ion auto-analyzer (Scalar SANplus segmented flow analyzer, the Netherlands). Olsen inorganic P was measured by extracting soil with 0.5 M NaHCO3 at pH 8.5. The HCl-P fraction was determined as described by Tiessen and Moir50. The available P referred to inorganic P which was calculated as the sum of Olsen inorganic P and HCl-P. Soil pH was determined in 1:2.5 (w/v) soil water suspensions using a glass electrode. Soil types, classified according to the FAO-UNESCO classification system, were derived from the Harmonised World Soil Database version 1.21 released in 2012 by FAO.

Climatic data

We used aridity for our modelling because it is a fundamental driver of biological and geochemical processes in drylands. The aridity index (AI) is the ratio of mean annual precipitation (MAP) to mean annual potential evapotranspiration (MAE). MAP values were obtained from the WorldClim Global Climate Data (htclass="Chemical">tp://WorldClim.org), for years 1950–2000, by usiclass="Chemical">ng R package ‘raster’. Poteclass="Chemical">ntial evapo-traclass="Chemical">nspiratioclass="Chemical">n (PET) layers were estimated oclass="Chemical">n a moclass="Chemical">nthly average basis by the Global-PET (htclass="Chemical">n class="Chemical">tp://www.cgiar-csi.org) and were aggregated to MAE. The AI decreases as aridity increases and AI is strongly related to climatic characters such as temperature and rainfall (Supplementary materials, Fig. S11). Our studied sites cover an aridity gradient from AI = 0.167 to AI = 0.604. We derived other climatic variables such as mean annual temperature from WorldClim Global Climate Data. Because soil nutrients bare more associated with current year climate than with mean climate, we derived the mean temperature and summed rainfall from May to August in 2013, i.e., the climatic variables in growth season prior to field sampling (ftp://ftp.ncdc.noaa.gov/pub/data/gsod/).

Statistical analysis

By using mean and standard errors, all soil variables at each site were averaged to obtain site-level estimates for statistical analysis. Probability of fit to normal distribution was tested by Shapiro–Wilk test. One-way analysis of variance was used to test the effect of soil types on soil C, N and P characteristics. When the difference was significant, post-hoc multiple comparisons were subsequently made using the Tukey–Kramer test. The relationships of independent variables (aridity index, longitude, latitude, altitude, growth-season temperature and precipitation in 2013) with the dependent variables (soil organic C, total N, total P, available N, available P, and their stoichiometric ratios of C:N, C:P and N:P) were explored by using either linear or curvilinear (quadratic) regressions. N:P ratios, calculated from total N and total P, or from available N and available P, were both considered in the analysis. To achieve normality, stoichiometric ratios of C, N and P were log-transformed before analyses. Multiple regression analysis with backward stepwise procedure was used to determine the best model (based on the Akaike information criterion [AIC]) and examine the overall patterns of response of soil C, N and P and their ratios to climatic (temperature and precipitation), soil pH and plant cover. Regression models were developed with increasing numbers of independent variables. The models labelled ‘T + R + ST + pH + PC’ include temperature, precipitation, soil type, soil pH and plant cover as explanatory variables (Table 2). For each of soil C, N and P and their ratios, two models were compared, including: 1) factors selected by the stepwise procedure, and 2) all factors. The values of the regression coefficients of the best model were used to compare the contribution of each variable to the soil variables (‘strength of effects’). The results were interpreted by examining the relative influence (or contribution) of each predictor from the best model based on MASS package. Statistical analyses were conducted with R 3.2.2.

Additional Information

How to cite this article: Jiao, F. et al. Increasing aridity, temperature and soil pH induce soil n class="Chemical">C-N-P imbalaclass="Chemical">nce iclass="Chemical">n grasslaclass="Chemical">nds. Sci. Rep. 6, 19601; doi: 10.1038/srep19601 (2016).