Organic carbon distribution and soil aggregate stability in response to long-term phosphorus addition in different land-use types

Understanding the relationships among phosphorus (P), soil organic carbon (SOC) and aggregate stability is vital for improving soil fertility and P and C cycling in different land-use types. Here, we assessed the responses of SOC distribution in different aggregate fractions to P addition in two different land-use types categorized in either a P-limitation or non-P-limitation status based on the stoichiometric ratios of microbial enzymes. Four different long-term fertilization managements (no fertilizer [control, non-P limitation]; nitrogen and potassium [NK treatment, P limitation]; phosphorus [P treatment]; and nitrogen, phosphorus, and potassium [NPK treatment]) were established in upland and paddy soils to compare the effects of pre-assessed non-P- and P-limited soil conditions in combination with different fertilizer additions. A wet-sieving method was used to obtain four sizes (> 2-, 0.25–2-, 0.053–0.25- and < 0.053-mm) of soil aggregates. The concentrations of SOC and iron and aluminum (Fe/Al) oxides in the forms extractable by DCB (Fed/Ald), oxalate (Feo/Alo) and polyphosphate decahydrate (Fep/Alp) were measured. The results showed that compared to the non-P-limited control, the P treatment reduced soil mean weight diameter (MWD) by 13.1% in upland and 9.7% in paddy soils. The MWD of NPK treatment was significantly higher by 24% than that of the P-limited NK treatment in upland soil, while the difference between the same two fertilization treatments was not significant in paddy soil. Compared with that of the control, SOC concentration in each size was greater in the P-treated upland soil, while in the paddy soil, SOC concentration of the >2-mm size were lower in the P and NPK treatments than in the control and NK treatments, respectively. Phosphorus addition decreased Fep/Alp of each aggregate size in upland soil, while it increased Fep/Alp of each aggregate size in paddy soil. To better quantify the effects of P on SOC and aggregate stability, we used a partial least squares path model (PLS-PM). The results indicated that P addition had a direct negative effect on MWD (−0.797, P < 0.01) under the non-P limitation condition (control vs. P treatment). The addition of P in upland soil had a high positive effect on MWD (0.565, P < 0.05) under the P-limitation condition (NK vs. NPK treatments), but the effect was not significant in paddy soil (0.047, P > 0.05). Thus, strategies aiming to improve soil carbon cycling and aggregate stability by regulating phosphorus addition should consider land-use type and soil P-limitation status.