Effects of Diversified Cropping Sequences and Tillage Practices on Soil Organic Carbon, Nitrogen, and Associated Fractions in the North China Plain

Abstract

Limited information is available related to soil organic carbon (SOC), nitrogen (N), and their associated fractions, especially in diversified cropping sequences with a combination of tillage systems. Therefore, a field study was conducted to evaluate the effects of cropping sequences and tillage systems on SOC and N and associated fractions. The experiment was comprised of two factors, i.e., (i) tillage systems: no tillage (NT) and rotary tillage (RT), and (ii) cropping sequences: wheat-soybean-wheat-maize (WSWM); wheat-maize-wheat-soybean (WMWS); wheat-soybean-wheat-soybean (WS); and wheat-maize-wheat-maize (WM). Tillage systems influenced the distribution of SOC and N and their associated fractions mainly at topsoil depth rather than deep soil, while cropping sequences affected SOC and N and their associated fractions differently in the whole soil sampling depth (0–50 cm). The results showed that NT had significantly higher SOC concentrations than RT at the 0–10- (17% higher) and 20–30-cm (19% higher) soil layers. Similarly, NT had 17% significantly higher N contents than RT at the 0–10-cm soil layer, but RT had 21% significantly higher N accumulation at the 10–20-cm soil layer. The particulate organic carbon (POC) was highest in WM and lowest in WS cropping sequence at 0–10-cm soil depth, while tillage did not affect POC distribution at 0–30-cm soil depth. Similarly, particulate organic nitrogen (PON) was significantly higher in soybean-included cropping sequences only at 0–10-cm soil depth. Some other fractions, such as dissolved organic carbon (DOC) and dissolved organic nitrogen (DON), were higher in soybean-included cropping sequences at 0–30- and 0–20-cm soil depths respectively. Mineral-associated organic carbon (MAOC) also increased by 28% and 34% (p < 0.05) under NT compared to RT at the 0–10- and 10–20-cm soil layers, respectively. In the case of cropping sequence comparison, WSWM had 30% higher SOC at the 10–20-cm soil layer than the other three cropping sequences. Notably, legume-included cropping sequences (WSWM, WMWS, WS) significantly increased N contents by 9%, 15%, and 22% and mineral-associated organic nitrogen (MAON) by 12%, 15%, and 17.5%, respectively, compared to the WM cropping sequence at the 0–10-cm soil layer. SOC and TN and their fractions were redistributed by tillage and cropping sequences at 20–50-cm soil layers. However, SOC stock was only affected by tillage systems (NT had 10% higher than RT) rather than cropping sequences. But WMWS and WS cropping sequences had 11% and 10% significantly higher N stock than WSWM and WM sequences, respectively. Overall, our findings suggested that NT especially with soybean could be a suitable practice to sequester SOC and N in the North China Plain.