Nitrogen cycling functional gene abundance and potential activity in novel perennial forage cropping systems in Rwanda

Abstract

Soil nitrogen (N) fertility in sub‐Saharan Africa is limited by a range of environmental and edaphic factors. Some tropical perennial forage grasses, such as Brachiaria sp., conserve soil mineral nitrate (NO ₃ ⁻ ) through biological nitrification inhibition. However, there is limited evidence for the field‐scale relevance of this approach under typical smallholder management regimes. We evaluated N cycling dynamics under four forage crops that included a locally scalable Brachiaria hybrid (CIAT 36087) and two farmer‐preferred fodder sources, Cenchrus purpureus (Napier grass) and Zea mays (maize). We also evaluated whether a perennial legume intercrop contributed to soil fertility or altered N cycling. We collected samples from 1 year‐old forage plantings at two locations in Rwanda across a 6‐month period that captured the transition between the rainy and dry seasons. We analyzed multiple fractions of mineral and organic nitrogen, potential nitrogen cycling activity, and organic carbon, and quantified 20 N cycle microbial functional genes. We found that annual maize, not Brachiaria , exhibited the lowest overall potential nitrification activity when averaged across timepoints, which was associated with reduced nitrifier gene abundance in one location. However, maize plots exhibited the greatest seasonal increases in potential nitrification activity and the largest declines in NO ₃ ⁻ ‐N between the dry and rainy seasons. While Brachiaria did not exhibit strong field‐scale evidence for BNI, forage planting treatment was predictive of potential nitrification activity. We conclude that perennial forages present an opportunity to enhance soil N retention during seasonal variations in soil moisture in rainfed tropical smallholder systems.