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Nitrogen
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The nitrogen gap in soil health concepts and fertility measurements
Soil nitrogen (N) often limits productivity in agroecosystems, prompting fertilizer applications that increase crop yields but can degrade the environment. Nitrogen's dual role in both productivity and environmental quality should center it in soil health frameworks. We use recent evidence to argue that N availability is an emergent property of the integrated soil biogeochemical system and is strongly influenced by plant traits and their interactions with microbes and minerals. Building upon this, we theorize that the sources of plant and microbial N shift across soil health gradients, from inorganic N dependence in ecologically simple systems with poor soil health to a highly networked supply of organic N in healthy soils; ergo, investments in soil health should increase ecological complexity and the pathways by which plants can access N, leading to more resilient nutrient supplies and yields in a variable climate. However, current N assessment methods derive from a historical emphasis on inorganic N pool sizes and are unable to capture the shifting drivers of N availability across soil health gradients. We highlight the need to better understand the plant-microbial-mineral interactions that regulate bioavailable N as a first step to improving our ability to measure it. We conclude it will be necessary to harness agroecosystem complexity, account for plant and microbial drivers, and carefully integrate external N inputs into soils' internal N network to expand the routes by which N from organic pools can be made bioavailable. By emphasizing N in soil health concepts, we argue that researchers can accelerate advances in N use efficiency and resiliency. -
Carbon and nitrogen transfer from litter to soil is higher in slow than rapid decomposing plant litter: A synthesis of stable isotope studies
Litter decomposability determines litter mass loss rate, but how it affects soil carbon (C) and nitrogen (N) storage remains elusive. We compiled data from 25 litter decomposition studies tracing the fate of C and N during decomposition using stable C and N isotopes. An average of 24% of C lost from decomposing litter was recovered in the soil independent of the decomposition stage and the experimental conditions. In contrast, a higher amount of N lost from decomposing litter was recovered in the soil in laboratory (80%) than in field (58%) experiments. The proportion of the total C and N lost that was transferred to the soil was higher for slowly than for rapidly decomposing litter types. Our results demonstrate substantial soil C and especially N input from decomposing litters and suggest that slowly decomposing litters favor soil C and N storage compared to more rapidly decomposing litters.