Effects of mechanical compaction on soil microbial activities and carbon emission of oasis agricultural soils in Xinjiang

The impact of structural deformations on soil properties controls its function as habitat. Soil compaction is widespread but tends to be most prevalent where heavy machinery is used in forestry and agriculture. Untimely traffic with heavy logging machinery often leads to irreversible damage of soil structure, restricting plant growth, reducing crop yield or decreasing the greenhouse gas emissions mitigation potential from the farmland. Avoiding soil compaction caused by agricultural management is a key aim of sustainable land management; however, limited understanding exists on how compaction affects the soil carbon process and microbial activities. Thus, we conducted a 120-day laboratory incubation to evaluate the effects of soil compaction mainly in agricultural soils on soil organisms and soil biological processes to identify relevant parameters which are helpful for assessing soil compaction from the soil biological and environmental point of view. We evaluated if threshold values of soil bulk density correspond to impacts on soil microbial fertility and environmental effect on soil. Our literature review showed that bulk density of Xinjiang farmland soil ranges from 0.85-1.60 g/cm3. In this study we used i) non-compacted reference 1.15 g/cm3 T1.15, ii) Treatment 1.30 g/cm3 (T1.30), where soil was the median value of bulk density, iii) T1.45, with was 75% of the value, iv) T1.60 was the maximum bulk density of the Xinjiang oasis farmland. In this study, we measured the soil carbon emission rate under different bulk density conditions using a Li-8100 automated soil CO2 flux system ( LI-COR, Lincoln，Nebraska，USA) from June to October 2016, and soil organic carbon (SOC), urease activity (URE), catalase activity (CAT), dissolved organic carbon (DOC), microbial biomass carbon (MBC), microbial biomass nitrogen (MBN) were also measured. The result showed that during the laboratory incubation time (0-120 d) treatment T1.15, T1.30, T1.45 and T1.60, MBC, MBN, URE and CAT decreased with the extension of the test period. Under different bulk density levels, soil MBC, MBN, URE and CAT increased rapidly and then decreased with the increase of soil bulk density, and was the highest when the bulk density was 1.45 g/cm3. Cumulative carbon emission from T1.15, T1.30, T1.45 and T1.60 treatments was 557.26, 653.48, 665.00 and 522.01 g/m2, respectively. There was a trend of increasing first and then decreasing with increasing soil bulk density, and the highest was T1.45. The correlation analysis showed that soil carbon emissions were significantly correlated with urease and catalase activity (P<0.05), and very significantly correlated with soil organic carbon, dissolved organic carbon, and microbial biomass carbon and nitrogen (P<0.01). The effects of compaction on soil microbial activities and carbon emissions were weighed. Combined effects of trade-offs and compactions on soil biological activity and carbon emissions revealed that soil carbon emissions were the highest when the microbial activity is the highest, and therefore, the maximum microbial activities will also produce greater environmental effects. In summary, mechanical compaction directly affected microbial activities and carbon emissions by changing soil bulk density and pore structure. Therefore, when the soil capacity of oasis farmland was greater than or equal to or less than 1.45 g/cm3, proper overturning or repression should be carried out to make the soil biologically active at the best level.