Grain yield and nitrogen cycling under conservation agriculture and biochar amendment in agroecosystems of sub-Saharan Africa. A meta-analysis

Soil nitrogen (N) is one of the most limiting factors affecting crop production in sub-Saharan Africa (SSA). Here we conducted a meta-analysis on the effect of climate smart agricultural (CSA) practices (conservation agriculture (CA) and/or biochar (BC)) application on: (1) soil nitrate-N (NO3-N), nitrous oxide (N2O) emission, biological N-2-fixation, percent of nitrogen derived from the atmosphere (%Ndfa), grain yield and nitrogen use efficiency (NUE), (2) the role of soil properties and regions on grain yield and N cycling under CA and/or BC biochar application; and (3) the relationship between inorganic N fertilizer and NO3-N, N2O emissions, NUE and grain yield. We synthesized 87 unique papers, from 15 countries in SSA with 1643 paired observations. On average across all studies, CA and/or BC significantly increased grain yield and NUE, compared to conventional practices. Residue retention resulted in a significant increase in soil NO3-N and N2O emission, compared to conventional practices. Our analysis further indicates that BC application significantly increased biological N-2-fixation, grain yield and NUE. Auxiliary soil parameters also affected grain yield and N cycling. Grain yield was significantly influenced by total organic carbon classes (TOC), whereby highest grain yield was recorded under CSA in soils with 0.5-1 % TOC, compared to soils with < 0.5 % TOC and > 1 % TOC. In addition, total nitrogen (TN) significantly affected the response ratio of CSA and conventional agriculture on N2O emission and biological N-2-fixation. N2O emission increased significantly in soils with < 0.05 % TN, while biological N-2-fixation increased significantly in soils with > 0.2 % TN. Increasing N fertilizer use significantly increased the response ratio of CSA and conventional agriculture on N2O and NO3-N while significantly reducing the response ratio of yield and NUE. The gap in yield and NUE between CSA and conventional agriculture practises was more pronounced at lower N rates of 0 kg ha(-1) and narrowed as N input increased to 120 kg ha(-1); this implies that, CSA offers more benefits compared to conventional agricultural practices under low N rates.