Fatigue Evidence of Soil Organic Matter in the Northeastern Mollisol Area of China.
Xudong Zhang and Hongtu Xie. Key Lab. of Terrestrial Ecological Process, Institute of Applied Ecology, Chinese Academy of Sciences, #72 Wenhua Road, Shenhe District, Shenyang, 110016, China
Sustainability of arable land is closely related to the quantity and quality of Soil Organic Matter (SOM). Therefore, soil degradation on upland necessarily means some changes in the quantity and quality of SOM and then this changes may result in fatigue of soil functioning. Hence, in order to reactivate soil functions of depredated arable land, we must understand how soil functioning fatigue is caused by SOM fatigue. In this context, it is critical to find some evidence of SOM fatigue and to evaluate whether the fatigue character can be converted by agricultural practice in terms of restoration of soil functions. Our objective was to investigate the fatigue evidence of SOM in the northeast of China and evaluate a reversal measure. Two sets of surface soil samples were used for the study. The first set of samples from 1450 sites was collected all over the Mollisol area in the northeast of China. The second set was sampled from a long-term research site of Mollisols. The treatments include individual or combined application of chemical and/or organic fertilizers for 25 years. After sampling, bulk soils were fractionated into 4 aggregate classes (<53, 53-250, 250-2000, >2000Ám). The contents of SOM, total N, and bio active organic carbon were determined by using conventional methods and C mineralization was measured by using an incubation technique. The data from the first set of 1450 samples showed that frequency distribution of SOC contents followed a non-normal pattern, i.e. the frequency at the higher content range was shifted to the lower content range significantly and this pattern was especially clear for the large size aggregates. This phenomena could be an evidence of SOM fatigue in the Chinese Mollisol due to over use of the soil (traditional tillage system without plant residue return), which resulted in depletion of labile (bio-active) part of SOM. However, the fatigue character could be converted by the application of organic manure, as indicated by evaluation of SOM quantity and quality data. The results from the long-term experiment revealed that amendment with organic manure for 25 years increased soil bioactive organic carbon by 20- 55% and the increment of bioactive organic carbon was mostly assigned into and finally restored in the large sized aggregates. The changes in the soil bioactive organic carbon were highly related to the use amount of organic manure. However, chemical fertilizer application did not affect the concentration of soil bioactive organic carbon significantly. The mineralization experiments showed that the calculated C0 (mineralization potential) of soil organic carbon from separated incubation of each aggregate did not change compared with the result of the mixed incubation of the 4 aggregates (or bulk soil incubation). Nevertheless, the mineralization process was sped up remarkably when individual aggregates were incubated compared with the mixed incubation data. The finding suggests that proper compositions and combination of different size aggregates are important for the protection of bioactive organic carbon from rapid mineralization. In conclusion, the non normal distribution pattern and rapid depletion of SOM, especially bio-active part in the Chinese Mollisols could be a fatigue evidence of SOM or soil functioning due to over use of the land upon traditional tillage system. However the fatigue character could be converted partly or totally by the continuous amendment of organic manure. The mechanism of SOM fatigue was likely due to disaggregating of soil large aggregates and structure, and this results in rapid depletion of bioactive organic carbon and, in turn, deteriorates soil functioning.