Nitrogen loss factors of nitrogen trace gas emissions and leaching from excreta patches in grassland ecosystems: A summary of available data.

Patches of excreta voided by grazing animals are nitrogen (N) transformation hotspots in grassland ecosystems and an important source of N trace gas emissions and leaching. Previous studies have focused on individual N losses from excreta, but no quantitative analysis has been performed on all the N losses via N trace gas emissions and leaching. To better understand the fate of N in excreta patches, we summarized 418, 15, 65, 22, 54, 11, and 81 measurements of nitrous oxide (N2O), nitric oxide (NO), ammonia (NH3), and ammonium (NH4+) leaching, nitrate (NO3-) leaching, dissolved organic nitrogen (DON) leaching, and aboveground plant N uptake, respectively. The results based on field studies indicated that the average fractions of N lost via N2O were 0.28%, 0.76%, 0.08%, and 0.35% for cattle dung, cattle urine, sheep dung, and sheep urine, respectively. Only 0.01-0.12% of excreta N was lost via NO, whereas 1.69-12.7%, 0-4.58%, 16.4-24.6%, and 1.43-5.91% were lost by NH3 and NH4+, NO3-, and DON leaching, respectively. Aboveground plant parts assimilated 10.4-31.4% of the excreta N. The N lost via N2O from urine patches decreased as NH3 losses increased, and greater NO3- leaching occurred with lower plant N uptake. The combined N2O emission factors for dung and urine from cattle and sheep were 0.59% and 0.26%, respectively. Each N loss factor was much higher in urine patches than in dung patches, irrespective of animal type. This study provides general estimates of N losses and plant N uptake from excreta patches on grazed grassland based on currently available field data. More field studies are needed in the future with longer measurement periods from a wide range of climate zones to refine these N loss factors.