Characteristics of Soil C:N Ratio and Δ13c in Wheat-Maize Cropping System of the North China Plain and Influences of the Yellow River

To better understand the characteristics of soil organic matter (SOM) in the North China Plain, we evaluate the large scale variations of soil organic carbon (SOC), total nitrogen (TN), carbon to nitrogen (C:N) ratio and stable carbon isotopic compositions (δ13C) in SOC over 0–100 cm. To assess the influence of the Yellow River, 31 sites are selected from the wheat-maize double cropping system, and grouped into two: 10 sites near and 21 sites far from the river. Our data show that mean soil C:N ratio is low (7.6–9.9) across the region, and not affected by the Yellow River. However, SOC and TN are significantly (P < 0.05) lower in subsoil near the Yellow River (2.0 and 0.2–0.3 g kg−1 for SOC and TN) than far away (3.1 and 0.4 g kg−1); δ13C is significantly more negative below 60 cm near the river (−23.3 to −22.6‰) than far away (−21.8 to −21.4‰). We estimate that the contributions of wheat and maize to SOC are 61.3–68.1% and 31.9–38.8%, respectively. Our analyses indicate that the overall low levels of SOC in the North China Plain may be associated with the low soil C:N ratio and less clay content. The hydrological processes may also partly be responsible, particularly for those near the Yellow River.