Retention of crop biomass is widely recommended to improve soil organic carbon (SOC). However, the magnitude of contribution of aboveground residues and belowground roots from C3 and C4 crops to SOC is unclear.

Data from a 10-year field experiment and a 60-day laboratory incubation were synthesized to identify the respective contribution of C3 (e.g., wheat) and C4 (e.g., maize) residues and roots to SOC, as well as its underlying mechanisms under no-till (NT) using ¹³C labelling trace in wheat-maize rotations.

The field experiment showed that residue retention significantly increased SOC accumulation, and SOC derived from wheat was 126.0% higher than that from maize. Conversion to NT promoted SOC derived from wheat and thus accumulated 17.6% higher SOC stock compared with plow tillage (PT) under residue returning at 0-20 cm soil depth (P<0.05). The data from laboratory incubation revealed the mechanisms that lower priming effects at 0-10 cm depth decreased total mineralization by 91.8% after inputs of wheat residues and roots compared with that of maize residues and roots, especially under NT compared with PT. Priming effects were negatively correlated with enzyme activities associated with the C recycle, SOC, and total nitrogen (TN) contents (P<0.01). NT increased enzyme activities, SOC, and TN contents and thus reduced priming effects and improved residual C.

Synthesis and applications. These results suggested that wheat may contribute more to SOC accumulation than maize, and carbon increment efficiency in farmland could be enhanced by considering the crucial roles of C3 crops in SOC accumulation. NT practice sustains the benefits of C3 crops to SOC sequestration in the upper soil depths.

2.1. Field experiment

In 2008, a field experiment was conducted at the Wuqiao Experimental Station of China Agricultural University, located in Hebei Province (Longitude of 37°36´N, Latitude of 116°21´E). Two tillage (NT and PT) and two residue managements [residue returning (RR) and residue removal (R0)] were combined into four treatments: no-till with residue retention (NTR), no-till with residue removal (NT0), plow tillage with residue incorporation (PTR), and plow tillage with residue removal (PT0) using a randomized block design. Each treatment was replicated three times with a plot area of 225 m² (15×15 m). Details of farm operations for experiment are presented in Table S1. Basic soil properties prior to experiment are presented in Table S2. Irrigation and fertilizer use were based on the local farmers' practice (Kan et al., 2020c). Wheat and maize were grown following an annual rotation cycle.

2.2. Laboratory incubation

Soil sampled from the field experiment during the wheat (June 2018) and maize season (October 2018) were incubated for 60 days in a growth chamber. Soil incubation was done at a temperature of 25 °C and a soil moisture content of 70% of water holding capacity (WHC). Soil moisture was measured over time through gravimetric methods, and distilled water was added to maintain at 70% WHC level. Labelled aboveground residues (δ¹³C=873.42‰ and 112.40‰ for wheat and maize) of wheat and maize were uniformly mixed at 3 g kg⁻¹ (sampled from wheat season) and 4 g kg⁻¹ dry soil (sampled from maize season), corresponding to 8.4 and 11.2 Mg ha⁻¹, respectively at 0–20 cm depth (bulk density: 1.4 g cm⁻³). Similarly, labelled belowground roots (δ13C=537.38‰ and 106.79‰ for wheat and maize) were mixed with soils following the same procedure as with the aboveground residues. Methods of residue and root labelling are presented in the Supporting Information. Soils without residues (or roots) addition were used as a control. Basic properties of wheat and maize residues are shown in Table 1. A CO₂ trap (NaOH) was used to absorb the CO₂, and the released CO₂ was measured on 5, 10, 20, 40, and 60 days after incubation. Carbonates were extracted by BaCl₂, where Na₂CO₃ was transformed to BaCO3 for δ¹³C analyses. The BaCO₃ was used to measure 13C abundance by ANCA-IRMS. The residual NaOH was titrated by HCl using phenolphthalein.