Abstract
This study aimed to investigate the soil organic carbon (SOC) sequestration rate and soil organic matter (SOM) composition in conventional rotational cropping with mineral fertilization compared with organic cover cropping with and without composted manure addition during 2008–2018 to specify the SOM stabilization under different farming systems. The SOC proportion in particulate organic matter (POM) (63–2000 µm) and mineral-associated organic matter (MAOM) (<63 µm) fractions were estimated in different treatments, and the SOM composition in the fractions was characterized by FTIR spectroscopy. The SOC sequestration rate was treatment-dependent, with the higher SOC sequestration rate (1.26 Mg ha−1 y−1) in the organic treatment with cover crop and composted manure. Across all treatments, 57.3%–77.8% of the SOC stock was in the MAOM fraction. Mineral N fertilization increased POM-C concentration by 19%–52% compared with the unfertilized control. Under the organic treatments, the POM-C concentration was 83%–95% higher than the control. The MAOM-C concentration increased by 8%–20%. The mineral N fertilization and organic treatments (with and without cover crops and composted manure) increased the SOC stock proportion of POM. The highest proportion of SOC stock related to POM was in the cover cropping system, reducing the proportion of C related to the MAOM fraction, but the addition of composted manure with cover cropping also increased the proportion of C in MAOM. Compared with MAOM, the POM had a less resistant organic matter composition, and the POM resistance was higher in organic than conventional treatments. In general, the recalcitrance of SOM increased with SOC concentration. The POM fraction had higher aromaticity (or degree of decomposition) than the MAOM fraction. The aromaticity in POM and MAOM fractions was higher in the organic farming system and depended on mineral N fertilization and cover cropping, but the effect of manure was not significant. Although the SOC sequestration rate was higher under manure addition, resulting in the highest formation of both POM and MAOM in the soil, manure addition had little effect on overall SOM composition compared with cover crops.
Highlights
Soil is the largest soil organic carbon (SOC) reservoir in terrestrial ecosystems [1]
We hypothesized that as the soil organic matter (SOM) formation processes vary between conventional and organic farming systems and the particulate organic matter (POM) and mineral-associated organic matter (MAOM) fractions are fundamentally different in terms of their properties, the SOM composition and stability depend on farming systems and farming systems affect POM and MAOM composition differently
In the O-control, cover crops were grown until 2011, which resulted in an increase in SOC concentration of 9.5%, but upon discontinuation of cover crops, the SOC concentration decreased to the level at the beginning of the experiment after the second crop rotation in 2018 (14.7 g kg−1)
Summary
Soil is the largest soil organic carbon (SOC) reservoir in terrestrial ecosystems [1]. A global meta-analysis reported that cover crops increase SOC stocks by 0.32 Mg ha−1 y−1 in the upper 22 cm of soil [4] These practices are important in organic farming systems where manure and cover crops help maintain soil fertility and crop productivity and enrich the soil with nitrogen (N) [10], in addition to reducing N losses via leaching [11]. The aim of this study was to (i) investigate the effect of different farming systems (organic vs conventional systems) on the SOC sequestration rate, (ii) study the fractional composition of soil organic matter in a long-term crop rotation experiment, (iii) characterize SOC and total N (Ntot) in POM and MAOM fractions, and (iv) describe the chemical composition of POM and MAOM fractions through functional groups using FTIR spectroscopy. We hypothesized that as the SOM formation processes vary between conventional and organic farming systems and the POM and MAOM fractions are fundamentally different in terms of their properties, the SOM composition and stability depend on farming systems and farming systems affect POM and MAOM composition differently
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