Incubation Study On Characteristics Of Carbon Mineralization From Loess Soil Amended With Wheat And Maize Straw

In recent years, carbon sequestration in soils by appropriate agricultural management practices has been recognized by many researchers. It has become known that characteristics, sequestration and emission mechanisms of carbon in soil do change under different strategies. Soil organic carbon （SOC） largely influences soil quality, carbon dioxide evolution and food security. It has crucial role in maintaining soil fertility, nutrient cycling and controls soil gas emissions. Loss of SOC has detrimental effects on soil physical, chemical and biological properties. Generally, carbon （C） loss from soil to atmosphere as CO₂ and other greenhouse gases （GHGs） is enhanced due to inappropriate field management practices. The control of depletion of SOC and release of GHGs has become an urgent scientific issue worldwide, including China. Agricultural soils can serve both as sink and source for sequestering atmospheric carbon through effective soil management strategies. The utilization of crop straw may hold a good promise for restoration of depleted SOC stocks and reduce CO₂ emission. Therefore there is a need to evolve best soil crop management measures with organic waste （straw） recycling. There are few researches on the straw C decomposition in the soil with reference to quantifying CO₂ evolution as a means of determining decomposition rate, especially in Guanzhong Plain area. Direct returning of straw to the field is being practiced to address the issue of replenishment of SOC, decreasing loss of soil fertility and CO₂ emission as well as its impact on the environment. Straw decomposition is governed by many biotic and abiotic factors. This study tried to reveal the comprehensive effect of nutrient, temperature, moisture and other soil characteristics on CO₂ emission and sequestration after straw is returned to croplands. In order to investigate the decomposition characteristics of crop straw added to the soil with C mineralization and CO₂ evolution, three consecutive laboratory incubation studies were carried out under controlled environment to simulate field conditions. The study has scientific value in terms of biological, ecological, social and economic benefits. The major findings from the studies were as follows:1. In order to investigate the effects of abiotic conditions on straw decomposition, this experiment was designed to reveal the decomposition characteristics of mixing maize straw with soil under different C/N ratios and soil moisture conditions. Maize straw was used as experimental material and an incubation experiment was carried out. And the experiment contained two factors which were nitrogen （N） rates （0.04, 0.08, 0.16, 0.32 g N kg-1 soil） and soil moisture （M） （55%, 70%, 85% and 100 % of field capacity）, the overall 16 treatments were incubated at 25℃for 53 days. The N rates are described as N1, N2, N3 and N4 （for N rate 1, 2 ,3 and 4） where as M levels are ML, MM, MH and MV for low, medium, higher and very higher moisture levels, respectively. The obtained results were as follows: （1） Soil moisture had significant effect on cumulative CO₂-C emission. The cumulative CO₂ emission amount increased with increasing soil relative water contents. The maximum cumulative CO₂-C emission was obtained under 100% of field capacity （Wv） treatment. When nitrogen application rates were 0.04, 0.08, 0.16 g N/kg soil （C/N ratios were 80, 40, 20, respectively） had significant effect on straw decomposition, indicating that the original soil nitrogen might have met the demand for straw decomposition under the current experimental condition. The cumulative CO₂-C emission significantly decreased at the highest N rate （0.32 g N kg-1 soil）, and decreased by 10.6% compared to the average cumulative of the other three N levels; The CO₂-C evolution rate of all treatments depicted the same features, namely, the rates were fast at the early stage, became slower at the middle stage, and then slowest and remained stable at the late stage of the incubation period. Averagely, about 38.5% of organic carbon containing in maize straw was mineralized and lost as CO₂-C. （2） The SOC content was highest when the relative water content was 85% （WH） combined with 0.16 g N kg-1 soil （N3）. Significant difference existed for the average contents of soil microbial biomass carbon （MBC） among various moisture conditions, and it was maximum when the relative water content was 100%; no significant effect caused by N rate was found for MBC. （3） Soil microbial biomass nitrogen （MBN） varied regularly under different water level and N rates. As the relative water content and N rate increased, the regular pattern of MBN decreased in the following order: WH > WV > WL > WM and N3 > N4 > N2 > N1, which increased to the maximum at N3 and WH, then went down afterwards. In conclusion, the combination of N3 and WH was the optimum moisture and N condition for maize straw decomposition, as well as the best treatment for the SCO accumulation.2. Wheat straw was used to study the effect of temperature and microbial inoculation （MI） on CO₂ evolution, SOC sequestration, as well as on nitrogen accumulation. The experiment contained 6 treatments combined with two factors of temperature （15℃and 20℃） and three levels of MI （0, 0.1%, and 0.3%）, and then arranged in completely randomized design in the incubation chamber during the incubation period and lasted for 75 days. The CO₂ evolution was monitored during entire incubation process, and soil C and nitrogen were determined after the incubation. The results revealed that, （1） MI had no significant effect on CO₂ emission; however, the temperature could significantly affect straw decomposition and nutrients release. Straw addition brought about 40% more CO₂ production compared to no straw addition. CO₂ emission was significantly higher at 20℃than that at 15℃. （2） The net accumulative CO₂ emission （only from straw decomposition） was decreased by 37.1%, while the net increase amount of SOC and microbial biomass carbon reached 260% and 949%, at 15℃than at 20℃, respectively. This indicated that lower temperature was beneficial to SOC sequestration. Temperature coefficient （Q10） ranged from 1.4 -1.7 and 1.3- 1.5 in soils with and without straw, respectively. （3） In comparison with 15℃and 20℃, soil microbial biomass N increased 262%, and soil total nitrogen and NH4+-N decreased by 100% and 18.4%, respectively, under the former condition. In addition, MBC and MBN increased by 33% and 37% at 20℃compared with that at 15℃.3. An incubation experiment was carried out with straw of maize and wheat to investigate the effect of straw placement ways and P fertilization on process of their decomposition. The experiment contained 8 treatments with two straw placement ways （incorporated and surfaced applied） and two rates of P fertilization （with and without addition of phosphorus）. The results showed that, （1） there were significant influences of two types of straws, two straw placement ways and two rates of P fertilization on the cumulative and rate of CO₂-C emission. CO₂-C cumulative emission increased 17% in incorporated treatment compared to surface mulching treatments during decomposition. The addition of straw and P fertilization increased CO₂-C emission. The percentage of straw carbon transformed into CO₂-C was 38.9% and 26.2% in incorporated treatment and surface treatment, respectively. （2） Compared with no P, P addition decreased SOC, MBC and MBN by 23.3%, 21.9% and 12.9%, respectively; but had no significant effect on soil inorganic nitrogen.Based on the previous studies on the factors affecting straw decomposition, we could make some conclusions as follows: （1） Reasonable moisture conditions and C/N ratio could not only be in favor of accelerating the straw mineralization process, but also increasing the content of soil organic carbon. （2） Lower temperature is favorable for SOC, MBC and MBN sequestration and retention, however, the higher temperature makes it easier to transform the organic carbon of straw into inorganic carbon, and in addition, microbial inoculation has little effect under both temperature regimes in treatments with and without straw. （3） Incorporation of the straw into soil could accelerate the decomposition process to be compared with mulching the straw on the soil surface. Straw decomposition not only need reasonable C/N ratio, but also demands phosphorus supply. In the other words, reasonable C/N/P ratios are necessary. In conclusion, 40% of organic carbon in straw would be lost as CO₂-C during straw decomposition process, regardless of wheat straw or maize straw. The rest of the organic carbon in straw may change into SOC and then supplement the soil carbon sink as carbon source, improve the soil quality, enhance organic recycling and transformation, as well as to balance the SOC in agro-ecosystem. Consequently, returning straw into fields and being timely decomposed in soil was a good measure to supplement soil carbon, nitrogen and some other soil characteristics, which was highly crucial to enhance soil fertility, improve soil quality and increase sustained crop production.