| Soil organic carbon (SOC) is the most important component in soil organic matter (SOM) composition, influencing soil aggregates formation and stability as well. Labile organic carbon is the active fraction in SOM, which is important to the processes of biochemistry, microbial metabolism for soil nutrient elements, and the process of SOM decomposition, etc. Maize straw carbon was one of the main exogenous organic carbon in the drylands of Northeast (NE) China, supplying and maintaining SOC for dryland soil. Nevertheless, there were some unclear questions such as how the SOC of dryland soils and aggregates was changing and how the addition of maize-derived carbon affected the transformation and fixation of new and native organic carbon. For these, this study was conducted on four long-term experimental sites (i.e. long-term Brown earth experimental site at Shenyang Agricultural University, long-term Black soil experimental site at Gongzhuling, long-term Black soil experimental site at Harbin and long-term Black soil experimental site at Hailun) in NE china, trying to trace the transformation and fixation of new and native organic carbon fractions (e.g. total organic carbon (TOC), water-dissolved organic carbon (WDOC), microbial biomass carbon (MBC) and particulate organic carbon (POC)) associated with fertility levels during 1-year decomposition period of maize straw carbon by using the 13C-labled maize straw combined with carbonrundum tube method. Additionally, we investigated transformation and fixation of new and native organic carbon fractions (TOC and MBC) in Brown earth and its water-stable aggregates under the plastic film mulching, which would help to provide research references on deepening the understanding of cultivated land SOC cycling, exploring its turnover process and mechanism of soil fertility immobilization. The main results were as follows:(1) The period of 0-60 days was the key period for maize straw carbon decomposition, and it was also the most active period for the dynamics of SOC fractions (e.g. TOC, WDOC, MBC and POC) of cultivated soil and its aggregates due to the disturbance of maize straw carbon decomposition.(2) Maize carbon contribution to TOC showed increasing trend from the north (black soil) to south (brown earth) in study areas, while its remaining showed decreasing trend. Meanwhile, it was higher in low fertility soil while lower in high fertility. By the effect of maize straw decomposition, the contribution or remaining of native-soil derived organic carbon to TOC was decreased from the north (black soil) to south (brown earth), showing it in cultivated soil was obviously higher than parent material, it in low fertility soil was higher than high fertility. The dynamics of native-soil derived organic carbon content indicated that maize carbon would be incorporated into native-soil derived organic carbon after 60 days, offsetting the loss of native-soil derived organic carbon before 60 days. In addition, the addition of maize carbon still increased TOC content and C stock of mulched and unmulched bulk brown earth, showing higher in unmulched bulk brown earth than mulched, and higher in high fertility than low fertility that was higher than parent material. Moreover, the remaining of native-soil derived organic carbon showed similar trend with TOC content among high fertility, low fertility and parent material, but it in mulched bulk brown earth was higher than unmulched, which suggested that cultivated soil more promoted the protection to native-soil derived organic carbon than parent material, and mulched soil more offset the loss of native-soil derived organic carbon that unmulched soil.(3) In the experimental period, WDOC fluctuated intensely, overall showing firstly increased then decreased. Meanwhile, the increase of it in black soil (north soil in study areas) was more than in brown earth (south soil in study areas) in both high and low fertility soil. For high fertility soil, the period of 0-180 days was the key period, in which maize carbon was more transferred to WDOC. Moreover, WDOC-δ13C value after addition of maize carbon showed that although maize 13C increased WDOC-δ13C due to it incorporation during 0-60 day, but the increase space was little (i.e. maize carbon contribution to WDOC was little), which suggested that the incorporation of maize 13C to WDOC was little, and its turnover in WDOC was fast, specially during 60-180 days. In addition, the addition of maize carbon obviously increased WDOC concentration in mulched and unmulched bulk brown earth with high or low fertility. The results of δ13C still indicated that maize 13C was weakly incorporated into WDOC. For the dynamics of native-soil derived WDOC concentration, it was more increased in unmulched soil than mulched, showing unmulched low fertility soil was more than high fertility, and the treatments of unmulching had an opposite pattern.(4) MBC increased and also fluctuated intensely by maize carbon addition, overall showing high fertility soil was higher than low fertility, and mulched soil higher than unmulched. Meanwhile, the effect of maize carbon addition on the dynamics of MBC of brown earth was in the early period (before 60 days), while that of black soil was in the later period (after 180 days). The results suggested that the conditions of high fertility and mulching would improve MBC activity. Moreover, the results of MBC-δ13C value indicated that maize 13C more incorporated into MBC than WDOC, showing that maize 13C tended to incorporate into MBC. Additionally, native-soil derived MBC concentration in mulched bulk brown earth showed increase trend in both high and low fertility soil in experimental periods, except that parent material showed firstly increase then decrease, which suggested that mulching would improve the incorporation ability of maize carbon into native-soil derived MBC. Compared with cultivated soil, parent material more weakly protected native-soil derived MBC due to the difference in nutrients and structure.(5) POC was increased by maize carbon addition, while showing a relative gradual fluctuation in all treatments (different soil types, fertility levels, and mulching and unmulching conditions). Meanwhile, maize carbon contribution to POC in low feriltiy soil was higher than high fertility. And in high fertility soil, this kind of contribution was higher in brown earth than in black soil. The results showed that unmulching and low feriltiy would promot POC increase. Moreover, the results of POC-δ13Cvalue indicated that the incorporation of maize 13C into POC was relatively obvious compared with WDOC, showing 13C was tended to incorporate into POC, with a dynamic pattern of 0-60 days fast increase, 60-180 fast decrease and then gradual. Overall, the addition of maize carbon increased and fixed native-soil derived organic carbon, becoming the basis of soil fertility improvement.(6) Maize straw 13C was selectively transferred and fixed in different SOC fractions, i.e. maize straw 13C, after maize straw decomposition, was more easily incorporated into the fractions of MBC and POC, while it was transfered in WDOC dominated by the form of 12C. Moreover, compared with MBC-13C and POC-13C, plastic film mulching would help to drive the maize carbon 13C forward to POC fraction, while maize carbon 12C forward to MBC fraction, which would increase the possibility of 12C pontential mineralization.(7) The formation of water-stable aggregates was associated with the different fertility levels of cultivated soil. With the improvement of soil tilth and fertility, macroaggregates by inner began to decompose to microaggregate (e.g. low fertility soil); and as the long-term influence by the farmland management measures (e.g. fertilization and plastic film mulching etc.), macroaggregates were reunited from microaggregates, thereby promoting the dynamics of water, fertility, air and heat. Subsequently, soil fertility was increased (e.g. high fertility or high fertility soil with plastic film mulching).(8) The addition of maize carbon decreased stability of water-stable aggregates of parent material, while increased that of high or low fertility soil. The stability of mulched soil was overall smaller than that of mulched soil, and that of low fertility soil was smaller than that of high fertility soil. Meanwhile, the stability of high or low fertility soil showed a trend of firstly increased by the period of 0-60 days and then decreased. The results suggested that whatever the treatments of high or low fertility, and mulching or unmulching, they affected the cementation ability of the product by maize straw decomposition and soil particles, and the aggregation would be decreased due to further decomposition of maize straw.(9) The addition of maize straw carbon increased TOC concentration, carbon stock and δ13C values in water-stable aggregates in all treatments. Meanwhile, TOC concentration and 13C were more enriched in the aggregate levels of≥2mm,0.25-1mm and 0.053-0.25mm, while carbon stock was mainly enrived in macroaggregates. The results showed that high fertility cultivated soil more promoted maize carbon decomposition, which decreased the amout of maize carbon incorporated into the aggregates. Meanwhile, plastic film mulching would also promote the decomposition of maize carbon decreasing the contribution ratio of maize carbon to TOC.(10) The addition of maize straw carbon increased MBC concentration in water-stable aggregates. Additionally, mulching and high fertility soils increased the disturbance of maize carbon in microaggregates, and in which enhance the transformation ability of MBC-13C. The results showed that with the increase of soil fertility, aggregate MBC in unmulching treatment would transfer from macroaggregate to microaggregate and then back to macroaggregat, while it in mulching treatment woulc concentrate in the level of 1-0.053mm during the period of 0-180 days. |
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