Response of soil carbon and nitrogen stocks to irrigation - A global meta-analysis.

Irrigation has profound influences on carbon (C) and nitrogen (N) stocks in agricultural soil. However, the global-scale irrigation effects on C and N pools in farmland soils, as well as the C: N ratio (C/N), remain unclear. This study integrates existing studies on C and N in irrigated farmland worldwide and investigates the responses of soil C and N concentrations, stocks, and the C/N to irrigation by meta-analysis. The results suggest that irrigation has a significantly positive impact on soil organic carbon (SOC) and total nitrogen (TN) stocks overall, with the stocks increase by 10.9 % and 7.4 %, respectively, and a 3.1 % increase in the C/N, but has no significant impact in soil microbial biomass carbon (MBC). The positive feedback of SOC (6.0 %) and TN (6.6 %) stocks in topsoil is more pronounced in response to irrigation than that in subsoil. The impact of irrigation on SOC stocks is greater in semi-arid regions and under flood irrigation. Furthermore, SOC stocks increase more in sandy and loamy soils compared to clay soil, while TN exhibits larger increases in clay soil. The results also indicate that the response of C/N to irrigation is more pronounced under the condition of deep soil, sandy soil, and semi-arid regions. The influence of irrigation on SOC stocks and the C/N increases with the duration of irrigation, while the impact on TN stocks tends to weaken. Our study deepens the understanding of the mechanisms behind irrigation's effects on soil C and N stocks and therefore provides theoretical insights for the management of soil fertility in irrigated agriculture.