Tue, Aug 03, 2021:On Demand
Background/Question/Methods
Microbial transformations of nitrogen (N) to leachable nitrate (NO3-) challenge soil fertility in agroecosystems. Nitrification also provides the substrate for denitrification by microbes, which is responsible for producing the potent greenhouse gas nitrous oxide (N2O). Some tropical forage grasses can alter these microbial N pathways through biological nitrification inhibition (BNI) via root-exudates that inhibit key steps in the nitrification pathway. Defoliation by manual harvesting affects below-ground soil microbial activity through changes in root exudation. Additionally, intercropping BNI-competent grasses like Brachiaria with fertility-enhancing leguminous forages may result in tradeoffs for reducing N-loss through BNI. To determine whether BNI can counteract the potential stimulatory effects of defoliation and legume intercropping on N loss, we collected soil samples from two replicated field trials in Rwanda. Each location included perennial forage grasses grown alone or intercropped with the legume Desmodium distortum. As the farmer-preferred annual crop, maize was included as a baseline to compare against the perennial forage treatments. We quantified several N fractions and assayed for nitrifier and denitrifier activity. Soil sampling occurred immediately before and two-weeks after complete defoliation for a period of six months that spanned both the rainy and dry seasons.
Results/Conclusions Preliminary results suggest that perennial forage grasses and legume intercropping impact bulk soil microbial activity and N mineralization. At one study location, both sole-cropped and intercropped stands of Brachiaria cv. Mulato II resulted in lower (p < 0.05) levels of denitrification enzyme activity compared to treatments that did not contain BNI-competent crops. In the second location, Brachiaria treatments resulted in lower (p < 0.05) denitrification activity compared to Pennisetum purpureum but not maize treatments. Interestingly, while competition between perennial grasses and legume intercrops resulted in reduced aboveground biomass compared to sole-cropped stands, legume presence did not stimulate nitrification or denitrification in all cases. Intercropping Brachiaria with D. distortum generally did not increase denitrification or nitrification potential activity, while including a legume intercrop enhanced denitrifier activity in maize treatments. Our results suggest that perennial forages interact with bulk soil microbial processes that lead to N loss in tropical agroecosystems. Furthermore, legume intercropping with BNI-competent grasses merits further study as a management strategy to address the dual goals of maintaining plant productivity and limiting agriculture’s environmental impact.
Results/Conclusions Preliminary results suggest that perennial forage grasses and legume intercropping impact bulk soil microbial activity and N mineralization. At one study location, both sole-cropped and intercropped stands of Brachiaria cv. Mulato II resulted in lower (p < 0.05) levels of denitrification enzyme activity compared to treatments that did not contain BNI-competent crops. In the second location, Brachiaria treatments resulted in lower (p < 0.05) denitrification activity compared to Pennisetum purpureum but not maize treatments. Interestingly, while competition between perennial grasses and legume intercrops resulted in reduced aboveground biomass compared to sole-cropped stands, legume presence did not stimulate nitrification or denitrification in all cases. Intercropping Brachiaria with D. distortum generally did not increase denitrification or nitrification potential activity, while including a legume intercrop enhanced denitrifier activity in maize treatments. Our results suggest that perennial forages interact with bulk soil microbial processes that lead to N loss in tropical agroecosystems. Furthermore, legume intercropping with BNI-competent grasses merits further study as a management strategy to address the dual goals of maintaining plant productivity and limiting agriculture’s environmental impact.