This study assessed specific peaks obtained by diffuse reflectance Fourier transform mid‐infrared spectroscopy (DRIFTS) for characterizing the soil organic matter (SOM) composition of a Haplic Chernozem. Soils were collected from the Static Fertilization Experiment, Bad Lauchstädt, Germany, during 5 years from the farmyard manure (FYM), mineral fertilizer (NPK), combination (FYM + NPK) and no fertilizer (Control) treatments. Soils were extracted with hot water (HWE), and fractionated by size and density. Bulk soil and fractions were analysed by DRIFTS. Peak areas at 2930, 1620, 1530 and 1159 cm−1 were selected as a range of organic functional groups (with limited mineral interference), integrated with a local baseline (corrected peak area) and each was divided by the summed area of the four peaks (relative peak area). Positive correlations between carbon (C) in fractions representing labile OM (<1.8 g cm−3, 1.8–2.0 g cm−3, CHWE) and the corrected peak area at 2930 cm−1 (3010–2800 cm−1) in the bulk soil indicated that this aliphatic peak corresponded to the more labile C compounds. Negative correlations between the same fractions and the corrected area of the predominantly aromatic peak at 1620 cm−1 (1660–1580 cm−1) in the bulk soil suggested a relationship with more stable SOM compounds. All relative peak areas were significantly affected by fertilizer treatment, with an increasing relative peak area at 2930 cm−1 in FYM compared with non‐FYM treatments. The ratio of the peaks at 1620 and 2930 cm−1 was positively correlated with the ratio of stable C (sum of C in >1.8 g cm−3 and clay fractions) to labile C (C content of <1.8 g cm−3 fraction) and thus taken as an indicator of SOM stability. The DRIFTS peak area method reflected changes in SOM quality and composition under long‐term management as measured by size and density fractionation, indicating heterogeneous chemical composition of the latter. Further, the DRIFTS analysis of undiluted soil samples can be used to assess SOM composition in small sample sets if specular reflection and mineral interferences are considered.

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Nitrogenous fertilizers have nearly doubled global grain yields, but have also increased losses of reactive N to the environment. Current public investments to improve soil health seek to balance productivity and environmental considerations. However, data integrating soil biological health and crop N response to date is insufficient to reliably drive conservation policy and inform management. Here we used multilevel structural equation modeling and N fertilizer rate trials to show that biologically healthier soils produce greater corn yields per unit of fertilizer. We found the effect of soil biological health on corn yield was 18% the magnitude of N fertilization, Moreover, we found this effect was consistent for edaphic and climatic conditions representative of 52% of the rainfed acreage in the Corn Belt (as determined using technological extrapolation domains). While N fertilization also plays a role in building or maintaining soil biological health, soil biological health metrics offer limited a priori information on a site's responsiveness to N fertilizer applications. Thus, increases in soil biological health can increase corn yields for a given unit of N fertilizer, but cannot completely replace mineral N fertilization in these systems. Our results illustrate the potential for gains in productivity through investment in soil biological health, independent of increases in mineral N fertilizer use. Since the Green Revolution, nitrogenous mineral fertilizers have helped to nearly double global grain crop yields 1. While this represents monumental gains in crop productivity, an estimated 41 to 50% of the N fertilizer applied to corn (Zea mays L.) globally since of the Green Revolution has been lost to the environment 2 , resulting in multifarious negative environmental effects 3-5. Many strategies to address N losses from cropping systems are centered on the management of N fertilizer (e.g., "4Rs" of fertilizer management 6), but neglect the role of soil 4,5 biology in supplying plant N which often supplies over 50% plant N uptake in a growing season 7-9. The framework of soil health seeks to highlight the critically important 10 , albeit inherently complex and uncertain 11,12 , role of soil biology in agroecosystems. Ultimately, soil health seeks to integrate soil biology with the historically emphasized chemical and physical soil components 13,14. Soil health has been widely embraced by farmers, researchers, and private industry alike 15-18. Additionally, soil health has also accrued broad legislative support in the form of nearly a dozen states incentivizing improved soil health as well as a Soil Health division within USDA 19. Buy-in from these stakeholders represents a nexus of several key sources of information growers use in making nutrient management decisions 20-22 , as well as a demonstrated financial commitment. While soil health has broad conceptual support, there is a dearth of empirical evidence connecting soil biological health measurements to vital soil functions or desired outcomes 23. Althou...

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