Effects of inhibitors and biochar on nitrous oxide emissions, nitrate leaching, and plant nitrogen uptake from urine patches of grazing animals on grasslands: a meta-analysis

Excreta (urine and dung) patches on grazed grasslands are significant sources of nitrogen (N) trace gas emissions and leaching. Nitrification inhibitors (NIs), urease inhibitors (UIs) and biochar have been tested to reduce N losses and increase N utilization in various agro-ecosystems. Although the effectiveness of NIs, UIs or biochar on N losses or N utilization for chemical N fertilizers and manures have been evaluated in previous studies, there has been no comprehensive assessment on their effectiveness for excreta patches of grazing animals on grassland. This study thus aims to better understand the effects of inhibitors and biochar on the major N dynamics in excreta patches. Thus, we analyzed the results of 44 studies (156, 65, 67, and 97 comparisons of N2O emissions, NO3- leaching, plant N uptake, and plant yields, respectively) to evaluate the effects of additives on N losses and uptake from excreta patches. Our results showed that compared with urine patches without additives, pyrazole derivatives (a NI), N-(n-butyl) thiophosphoric triamide (NBPT, a UI), and biochar did not affect N2O emissions, whereas dicyandiamide (DCD, a NI) and a combination of NBPT and DCD (NBPT + DCD) significantly reduced emissions by 51% and 48%, respectively. DCD and NBPT + DCD also significantly reduced NO3- leaching (46% and 42%, respectively), and increased plant N uptake (14% and 15%, respectively) and plant yields (7% and 12%, respectively). Our findings suggest that the application of DCD was effective in decreasing N losses and increasing N utilization from urine patches, but NBPT + DCD would be a better option, in order to avoid the potential increases in ammonia emissions following DCD application. However, the potential adverse impacts and economic viability of using the inhibitors should be evaluated fully before their large-scale application.