The Structure And Turnover Characteristics Of Mollisol Organic Matter Under Long-term Continuous Cropping

It had reported that increasing the content of organic matter fixed in soil played a beneficial role in repairing degraded soil, improving soil fertility and production, mitigating greenhouse effect. So researching stable mechanism of soil organic matter（SOM） became a focus in soil organic matter field. Chinese northeast Mollisol degradation problem is serious in recent ten years. And since new maize species introduction, maize economic benefits was great, which leaded to maize cropping area expanding in recent years. However, it is not clear that how will soil organic matter change under long-term continuous cropping. So it is urgent to study soil organic matter under continuous cropping systems, which can supplement the theory of SOM dynamic change, and elucidate stable mechanism of SOM under long-term monotonous cropping, provide theoretical basis for choosing better cropping systems. This study investigated the effects of long-term continuous cropping on soil organic matter structure and turnover based on long-term field experiments in Hailun Agro-ecology Field Research Station, Chinese Academy of Sciences. Based on aggregate class and density, SOM were separated into different fractions. Fourier Infrared Spectrometer, solid 13 C NMR and stable isotope skills were used to investigate SOM structure and turnover. The main results and conclusions are as follows:（1）The content of SOM and its fractions decreased in plough layer soil. Compared with rotation, soil organic matter content significantly decreased in all classes aggregates, and the greatest decreasing degree in micro-aggregates under contunuous cropping. After long-term maize continuous cropping, soil organic matter content tended to decrease in all aggregates, and decreased significantly by 5.85% in micro-aggregate. For all density fractions, light fraction carbon contents had significant change, increased by 32.74% in free light fraction, decreased by 16.72% in occluded light fraction, while no change in mineral-associated fraction.（2）Long-term continuous cropping made soil organic matter structure aliphatic and simple in plough layer. Compared with rotation, the relative content of aliphatic-CH, polysaccharide C-O,-OH in phenolic and alcohol increased, while aromatic C=C, C=O decreased under continuous cropping. After long-term maize continuous cropping, the relative content of aliphatic-CH, polysaccharide C-O, phenolic alcohol-OH increased, while the one of aromatic C=C and carboxyl C=O decreased, the ratio of-CH/C=C increased. The ratio of-CH/C=C mainly increased in > 2 mm, of which-CH/C=C value of three density fractionations increased, especially in LF; while it had increasing trend in the density fractions of other class aggregates. Principal component analysis also showed that SOM structure in >2 mm aggregate changed in the highest degree in all aggregates classes.（3）The relative intensity radio of characteristic peaks of alcoxyl and aromatic carbons increased under long-term continuous cropping, and SOM stability decreased. Compared with rotation, the relative content of SOM stable structure was less under continuous cropping in plough player. As soil depth deepen, SOM stability strengthened; compared with 0²0cm soil, the degree of increasing in 40⁶0 cm was largest under continuous soybean cropping, the radio of aromatic carbon and alcoxyl carbon increased by 60.22%; while 80¹00 cm under continuous maize cropping, the radio increased by 80.95%. The increasing degree of soil organic matter stability under continuous soybean cropping was higher than that under continuous maize cropping in upper soil; while in lower soil, about 80¹00 cm, organic matter stability decreased under continuous maize cropping.（4）Different crops had different effects on SOM δ¹³C, turnover rate. In plough soil, δ¹³C under maize was higher, while SOM turnover rate slower than soybean. Soil δ¹³C and SOM turnover rate in macro-aggregate were higher than in micro-aggregate under maize and soybean, and there was largest difference in >2 mm aggregates. In different deep profiles, compared with the ones before setting experiments, δ¹³C increased from 0¹.8 m in soil profile after long-term continuous maize cropping; while δ¹³C decreased from 0⁰.8 m in the profile after long-term continuous soybean cropping. Soil organic matter turnover rate was higher under continuous soybean cropping than continuous maize cropping.Long-term monotonous cropping decreased SOM content, and made SOM structure simple and aliphatic. Different crops had different effects on SOM turnover, which was lower under maize（C4 crop） than soybean（C3 crop） in monotonous cropping system. SOM structure and turnover rate were different in different deep profile, as soil profile deepen, SOM stability strengthened, while turnover rate decreased.