Manure application accumulates more nitrogen in paddy soils than rice straw but less from fungal necromass

Abstract

Microbial processes involving in nitrogen (N) assimilation mediate the continuous supply of mineral N for crop growth and the reactive N loss in intensively managed agriculture soils. This study aimed to reveal the microbial assimilation role in N accumulation in paddy soils under partial substitution of mineral fertilizer with organic materials (straw or manure), by quantifying the microbial N derived from living biomass and necromass. Rice rhizosphere and bulk soils were collected at a 31-year field trial including five fertilization treatments: no fertilizer (Control), mineral fertilizer alone (NPK), 20% mineral fertilizer substituted by rice straw (NPKS), and 30% and 60% mineral fertilizer substituted by manure (NPKLM and NPKHM, respectively). Incorporation of organic materials increased the total soil N by 22–76% compared to NPK, with a greater increase under manure- than rice straw-treated soils. Both organic inputs enhanced the amounts of living biomass and necromass N by 1.1–1.5 times via stimulating the activity of N cycling genes (chiA, AOA, narG, nirK, nirS, nosZ, nifH) in bulk soils. Although the pools of living biomass and necromass N in soils with manure amendments were 12–34% and 13–22% higher, than those with rice straw incorporation, their corresponding contributions to total N were 7.2–7.4% and 6.2–15% lower, respectively, suggesting a weaker relative microbial contribution in soil N accrual with manure amendments. Rice rhizosphere processes strengthened the role of microbial participation in soil N accumulation, particularly with organic inputs. The relative contribution of fungal necromass N to total N as well as the ratios of fungi to bacteria in both living biomass and necromass were lower under manure amendments than rice straw incorporation. As a whole, as compared with the incorporation of rice straw, the greater N accumulation in paddy soil with manure amendments is associated with the relatively weaker accumulation of fungal necromass and stronger retention of organic debris without microbial processing.