Abstract
Limited annual precipitation and increasing scarcity of irrigation water are forcing farmers in arid and semi-arid regions to convert irrigated fields into dryland farming, yet their impacts on soil carbon (C) and nitrogen (N) fractions are not fully understood. This study evaluated changes in soil C and N fractions and forage production under four cropping systems after irrigation retirement in a semi-arid condition. Treatments included an annual winter wheat forage (Triticum aestivum L.) (AWW), a perennial wheat (PW) forage (T. aestivum L. x Thinopyrum spp.) (PW), a mixture of native grasses (NG), and a pasture crop (PC = AWW and NG mixture). Evaluation of various soil C and N fractions, forage dry matter yield, and nutritive value parameters suggested that soil inorganic N and potentially mineralizable N (PMN) were 38%-45% and 36%-44% greater in autumn 2020 and 62%-68% and 46%-55% greater in spring 2021 under AWW and PW, respectively, than NG and PC. Soil potentially mineralizable carbon (PMC) was 58%-129% and 64%-138% greater under NG and PW than AWW and PC. The responses of other soil C and N fractions varied with soil sampling dates but not with crops or crop x sampling date interaction. Total forage dry matter yield was 4016, 7849, 12,517, and 13,869 kg ha-1 for AWW, PW, PC, and NG, respectively, and PW had the best forage nutritive value. These results suggest soil C and N mineralization could be enhanced by adopting perennial systems, including NG, PW, and PC, while PW maintains good-quality forage production.