[Carbon Cycling Processes in Croplands and Their Quantification Methods].

Recent studies have shown that the source of soil carbon(C) includes not only the input of crop C(rhizodeposit- and residue-C) to soil organic C(SOC) but also the contribution of soil autotrophic microorganisms to SOC and the fixation of soil inorganic C(SIC) from the soil inorganic chemical pathway and microbial biomineralization pathway. The level of SOC in croplands is mainly controlled by the balance between the input of crop C and the loss of SOC via decomposition. In the short term, the input of crop C usually promotes the SOC decomposition, showing a positive(rhizosphere) priming effect. We analyzed the literature on the rhizosphere priming effect of major crops and the priming effect of straw additions and found that they were on average 75% and 67%, respectively. The residual straw C in the soil could completely compensate for the SOC loss caused by the priming effect of straw returning. In croplands, rhizodeposit- and residue-C often coexisted, which resulted in at least three C sources(rhizodeposit-, straw-, and soil-C) for soil C input and output. Finally, we proposed a new method to distinguish the contribution of multiple C sources to the CO2 emission and the SOC input in rhizosphere soils, as well as the contribution of inorganic chemistry and microbial pathways to the SIC input in calcareous soils. This review is helpful to improve the understanding of the input and output pathways of SOC and SIC in croplands and to improve the accuracy of soil C assessment in croplands.

[How Different Ratios of Straw Incorporation to Nitrogen Fertilization Influence Endogenous and Exogenous Carbon Release from Agricultural Soils].

The adjustment of the C/N ratio by straw combined with fertilizer nitrogen (N) not only affects straw decomposition but also affects soil organic carbon (SOC) decomposition, i.e. the priming effects. Therefore, it is doubly important to study how the ratios of straw to N fertilizer influence the release of endogenous and exogenous C for greenhouse gas emission reduction and soil fertility improvement. We conducted a 32-week laboratory incubation experiment with 13C labeled maize straw under different N levels in farmland soil collected from fields in Huantai County to investigate the effect of the ratios of straw to N fertilizer on straw decomposition and the priming effects. Four treatments were set up, including CK, corn straw (S), corn straw+low urea rates (SN1), and corn straw+high urea rates (SN2). Dynamic sampling was conducted during the early stage (0-10 d), the middle stage (11-43 d), and the later stage (44-224 d) of straw decomposition. The approach was based on using a two-source mixing model to differentiate two sources of soil CO2 (straw and soil-derived C). With an increase in the incubation time, the contribution of SOC decomposition to soil CO2 emissions first decreased and then increased. On the contrary, the contribution of straw mineralization to soil CO2 emissions first increased and then decreased. By the end of the incubation time, the contribution of SOC and straw decomposition to soil CO2 emissions was 0.84-0.86 and 0.14-0.16, respectively. Over the whole incubation period, the effects of N fertilization on straw decomposition first increased and then decreased. The promotion degree of high and low N fertilization on straw decomposition was up to 15.8% and 7.9%, respectively. Over the whole incubation period, the inhibition degree of low N fertilization reached up to 7.1%, while high N fertilization showed a slight promotion trend of 0.7%. Therefore, the regulation of C:N by straw combined with fertilizer N not only affected the contribution of exogenous straw to SOC but also influenced the decomposition of endogenous SOC, and then influenced soil C fixation. Over the whole incubation period, straw C retention could not compensate for CO2 released by the priming effects, which led to a net loss of SOC.

[Estimation of Winter Wheat Photosynthesized Carbon Distribution and Allocation Belowground via 13C Pulse-labeling].

Evaluating the allocation of carbon (C) photosynthesized by winter wheat belowground is essential for C sequestration in soil and crop production. During the four growth stages of winter wheat, i. e., tillering, elongation, anthesis, and grain-filling, the method of 13CO2 pulse-labeling for the wheat was adopted. Destructive samplings were undertaken at 28 d after each labeling and the total C and 13C contents of shoots, roots, soil, and rhizosphere respiration were determined. Results showed that the majority of the fixed 13C was recovered in the aboveground (straw and grain), ranging from 51.6% to 90.8% in all growth stages. The allocation of 13C photosynthesized belowground (roots, soil, and rhizosphere respiration) decreased as the wheat growth advanced, while the 13C transferred to the aboveground increased. Of the total 13C input belowground, 22.9%-65.3% was respired by the rhizosphere, 24.3%-59.3% remained in the roots, and 10.4%-17.8% was incorporated into the soil organic carbon by rhizodeposition. Respired 13C within the last 2 d of the whole chase period (28 d) only accounted for 0.7%-2.7% of the total respired 13C, indicating that 28 days were long enough to ensure a complete distribution of photosynthesized C within all the wheat and soil pools. For the whole growth season of winter wheat, the photosynthesized C allocated aboveground, to roots, soil organic carbon, and rhizosphere respiration was 78.5%, 6.0%, 3.1%, and 12.4% of the net assimilated C, respectively. Based on local wheat production, the total C transferred belowground was quantified as 1.72 t·hm-2, with 0.99 t·hm-2 respired as rhizosphere respiration, 0.48 t·hm-2 retained in roots, and 0.25 t·hm-2 incorporated into soil organic carbon.