Abstract
Abstract. Long-term (10 years) application of conservation tillage following conversion from conventional tillage (CT) can achieve a new equilibrium in the soil environment, which is vital to reverse soil biodiversity declines and fulfil the goal of maintaining agroecosystem sustainability. However, in such a situation, how the soil community regulates nutrient cycling impacting crop yield is not well documented. Therefore, the relations between mineralized nitrogen (N) delivered by soil food web and soybean (Glycine max Merr.) yield were investigated after 14 years application of CT, reduced tillage (RT) and no tillage (NT) in a black soil (Typic Hapludoll) of Northeast China. We hypothesized that soil mineralizable N would increase with the complexity of the soil food web, and that the trophic groups involved in associating N mineralization with crop yield will vary with soil depth in the conservation tillage practice. During the soybean growing season, soil organisms, including bacteria, fungi, nematodes, mites and collembolans, were extracted and identified monthly from 0–5 and 5–15 cm soil depths to estimate the complexity of the food web indicated by the species richness and connectance indices, and to simulate the mineralized N using energetic food web modelling. The species richness and connectance of the food web at both soil depths were significantly affected by tillage practices, and their values decreased of the order of NT > RT > CT. A similar trend was also revealed for the simulated N mineralization, that is, the mineralized N released either from the functional feeding guilds or from the energy pathways of the food web were greater in RT and NT than in CT at both soil depths. Multiple linear regression analysis showed that soil organisms involved in coupling the mineralized N with soybean yield were different at different soil depths, in which fungal and root pathways at 0–5 cm and bacterial pathway at 5–15 cm were the driving factors for the supply of mineralized N to soybean in NT and RT soils. These results support our hypothesis and highlight the essential role of soil food web complexity in coupling N mineralization and crop yield after long-term application of conservation tillage. Additionally, the current modelling work provides basic hypotheses for future studies to test the impact of soil biodiversity or specific functional guilds on the fate of N in agro-ecosystems.
Highlights
Nitrogen (N) is the most important growth-limiting nutrient for crops (Fageria et al, 2010)
The results showed that the variation pattern in the soybean yield among different tillage systems is counter to the empirically observed soil mineral N (Table 1), which was lower in reduced tillage (RT) and no tillage (NT) than in conventional tillage (CT), at both the lower soil layer (5–15 cm) and at the entire layer (0–15 cm), but is consistent with the simulation of mineralizable N pool (Fig. 2 and Table S7) that decreased of the order of NT > RT > CT at both soil depths
4.2 Tillage effects on the N mineralization within the food web Consistent with our first hypothesis, the results showed that, as the structure of soil food web became more complex after the conversion from conventional tillage to conservation tillage, mineralized N released either from the functional feeding guilds or from the energy pathways of the food web was greater in RT and NT than in CT at both soil depths
Summary
Nitrogen (N) is the most important growth-limiting nutrient for crops (Fageria et al, 2010). Crop production, to a great, extent relies on N mineralization to meet the growth requirements (La Menza et al, 2020; Whalen et al, 2013). The predators usually have a higher C : N ratio than their prey, which results in more N obtained than their nutritional requirements, and the excess N is excreted into the soil ammonium (NH+4 ) pool (de Ruiter et al, 1993; Whalen et al, 2013). Several studies (Bender and van der Heijden, 2015; Thakur et al, 2014; Wagg et al, 2014) based on controlled (micro- or mesocosm) experiments demonstrated that the potential mineralizable N pool increases with the increase in complexity of the food web, which implies that a management practice that forms a complex soil food web is beneficial for improving N availability with less N fertilizer input
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