Stabilization of soil aggregate and organic matter under the application of three organic resources and biochar-based compound fertilizer

Purpose Biochar mediation of soil structure and carbon (C) sequestration has been tested, but the underlying mechanisms in this process are poorly understood. We investigated the effects of different organic matter applications on soil aggregation and associated C in Northeast China. Materials and methods Non-disrupted soil samples (0–20 cm in depth) were collected from the field improvement experiment (established in 2009) with four addition treatments: (1) corn straw (CS), (2) pig manure compost (PMC), (3) biochar from corncob (BIO), and (4) biochar-based compound fertilizer (BF), of which CS, PMC, and BIO were combined with extra mineral fertilizers including nitrogen (N), phosphorus (P), and potassium (K). Soil aggregate distribution and associated C concentrations were estimated using physical fractionation techniques. Results and discussion The CS and PMC additions increased mean weight diameter (MWD), large macroaggregate fractions (>2 mm), and small macroaggregate fractions (0.25–2 mm). In contrast, BIO and BF significantly increased silt + clay fractions (< 0.053 mm). Compared with CS and PMC, applying BIO and BF had limited benefits for soil aggregation. Nevertheless, BIO promoted fine and coarse particulate organic matter (POM) accumulation significantly, and BF promoted fine POM accumulation significantly (within small macroaggregates). The concentrations of mMPOM (POM occluded in microaggregates within small macroaggregates) in BF and mPOM (POM within microaggregates) in BIO were the highest ( P < 0.05). Conclusions Biochar addition did not significantly improve soil structure compared to corn straw and pig manure compost treatments, but it was beneficial to SOC accumulation. Furthermore, C sequestration in the organic addition treatments was driven by the POM dynamics, and biochar debris was preferentially incorporated into the < 0.25 mm fractions.