Both soil organic carbon (SOC) and iron (Fe) oxide content, among other factors, drive the formation and stability of soil aggregates. However, the mechanism of these drivers in greenhouse soil fertilized with organic fertilizer is not well understood. In a 3-year field experiment, we aimed to investigate the factors which drive the stability of soil aggregates in greenhouse soil. To explore the impact of organic fertilizer on soil aggregates, we established four treatments: no fertilization (CK); inorganic fertilizer (CF); organic fertilizer (OF); and combined application of inorganic and organic fertilizers (COF). The application of organic fertilizer significantly enhanced the stability of aggregates, that is it enhanced the mean weight diameter, geometric mean diameter and aggregate content (%) of >0.25 mm aggregate fractions. OF and COF treatments increased the concentration of SOC, especially the aliphatic-C, aromatic-C and polysaccharide-C components of SOC, particularly in >0.25 mm aggregates. Organic fertilizer application significantly increased the content of free Fe (Fed), reactive Fe (Feo), and non-crystalline Fe in both bulk soil and aggregates. Furthermore, non-crystalline Fe showed a positive correlation with SOC content in both bulk soil and aggregates. Both non-crystalline Fe and SOC were significantly positively correlated with >2 mm mean weight diameter. Overall, we believe that the increase of SOC, aromatic-C, and non-crystalline Fe concentrations in soil after the application of organic fertilizer is the reason for improving soil aggregate stability.
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The effects of different straw returning patterns on soil carbon dioxide (CO2) emission characteristics and carbon balance were discussed, which provided a scientific basis for carbon (C) sequestration and emission reduction as well as the selection of straw returning patterns in Northeast China.
A field micro-plot experiment were conducted with maize as the experimental crop, and three straw returning patterns were set up, including straw shallow returning (QH), straw deep returning (SH), and straw mulching (FG). No straw returning (CK) was used as the control treatment. The LI-8100A automatic soil C flux tester was used to monitor soil CO2 emission characteristics under different straw returning patterns during the maize growth period. Effects of soil temperature, soil moisture content, pH, MBC, available nutrients and total nutrients of nitrogen, phosphorus and potassium on soil CO2 emissions were analyzed, and soil carbon balance was investigated too.
During the maize season, soil CO2 emission rates showed a trend of first increasing and then decreasing under different straw returning patterns. The cumulative soil CO2 emissions were as follows: FG>QH>SH>CK treatment. Compared with SH treatment, the cumulative soil CO2 emissions under FG and QH treatments increased by 14.0% and 6.4%, respectively. There was a significant difference between the treatments (P<0.05). The single factor model fitting of soil CO2 emission rates, soil temperature and soil moisture content under different straw returning patterns showed a quadratic function correlation, and reached a significant level (P<0.05), soil temperature could explain the variation of soil CO2 emission rate of 68.2%-73.7%, and soil moisture content could explain 21.3%-37.5%. However, the two-factor composite model of soil temperature and soil moisture content could better explain the variation of soil CO2 emission rate, with an explanation of 78.5%-82.8%. Correlation analysis showed that cumulative CO2 emissions were significantly correlated with available potassium and MBC (P<0.01), and significantly correlated with soil organic matter, available nitrogen, total nitrogen, and pH (P<0.05). The soil carbon balance was positive under different straw returning patterns, which were the "sink" of atmospheric carbon dioxide. The soil carbon balance and carbon sequestration potential under the SH treatment were significantly higher than the QH and FG treatments by increased of 23.4%, 475.7% and 7.1%, 30.7% (P<0.05), respectively. Compared with other treatments, the SH treatment showed a strong carbon "sink" function. In the two-year harvest period, straw returning treatments significantly increased maize yield, SH treatment had the highest maize yield, but there was no significant difference with QH and FG treatments.
Therefore, under the conditions of this experiment, taking into account the carbon sequestration and emission reduction effect and yield, SH was a better straw returning pattern compared with the three patterns.
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