Topsoil organic carbon mineralization and CO_2 evolution of three paddy soils from South China and the temperature dependence

Carbon mineralization and its response to climatic warming have been receiving global attention for the last decade. Although the virtual influence of temperature effect is still in great debate, little is known on the mineralization of organic carbon （SOC） of paddy soils of China under warming. SOC mineralization of three major types of China＇s paddy soils is studied through laboratory incubation for 114 d under soil moisture regime of 70% water holding capacity at 20℃ and 25℃ respectively. The carbon that mineralized as CO2 evolved was measured every day in the first 32 d and every two days in the following days. Carbon mineralized during the 114 d incubation ranged from 3.51 to 9.22 mg CO2-C/gC at 20℃ and from 4.24 to 11.35 mg CO2-C/gC at 25℃ respectively; and a mineralizable C pool in the range of 0.24 to 0.59 gC/kg, varying with different soils. The whole course of C mineralization in the 114 d incubation could be divided into three stages of varying rates, representing the three subpools of the total mineralizable C： very actively mineralized C at 1-23 d, actively tnineralized C at 24--74 d and a slowly mineralized pool with low and more or less stabilized C mineralization rate at 75-114 d. The calculated Q10 values ranged from 1.0 to 2.4, varying with the soil types and N status. Neither the total SOC pool nor the labile C pool could account for the total mineralization potential of the soils studied, despite a well correlation of labile C with the shortly and actively mineralized C, which were shown in sensitive response to soil warming. However, the portion of microbial C pool and the soil C/N ratio controlled the C mineralization and the temperature dependence. Therefore, C sequestration may not result in an increase of C mineralization proportionally. The relative control of C bioavailability and microbial metabolic activity on C mineralization with respect to stabilization of sequestered C in the paddy soils of China is to be further studied.

Effects of soil moisture and soil depth on nitrogen mineralization process under Mongolian pine plantations in Zhanggutai sandy land, P. R. China

The rates of soil N mineralization at soil depths of 0-15, 15-30, 30-45 and45-60 cm and moisture regimes were measured at three sand-fixation plantations of Pinus sylvestrisvar. mongolica by laboratory aerobic incubation method. The results showed that average rates ofsoil net N-mineralization across soil depth varied from 1.06 to 7.52 mg · kg^(-1)·month^(-1) atsoil depths from 0 to 60 cm. Statistical analyses indicated that the effects of different soildepths, moistures and their interactions on net N-mineralization rates were significant (P < 0.05).The net N-mineralization rates significantly decreased with increasing soil depths and at depth 0-15cm accounted for 60.52% of that at depth of 0-60 cm. There was no difference in soil netN-mineralization rates between half and fully-saturated water treatments, however these rates weresubstantially higher than that without water treatment (P < 0.05). The factors influencing Nmineralization process have to be studied further in these semiarid pine ecosystems.

Impacts of increasing maize stalk retention amount on soil respiration and temperature sensitivity

Conservation tillage with maize stalk retention is an effective method to replenish soil nutrients.Nutrient availability plays a major role in the control of soil respiration(SR).However,it is not known how different degrees of maize stalk retention control SR and its temperature sensitivity(Q_(10)).To investigate the effect of maize stalk retention amount on SR and Q_(10),four maize(Zea mays L.)stalk retention treatments,including(i)control treatment(CT)without maize stalk retention,(ii)standing maize stalk retention(SCR),(iii)partial maize stalk retention with‘three-year cycle’(TYR)and(iv)chopped maize stalk retention(CCR)was set up.In order to investigate the differences in soil nutrient,soil organic carbon(SOC)quality and soil microbial biomass among four treatments,soil analysis with 6 replicates was conducted.The experimental results showed that SR rates were 1.07,0.88,0.59 and 0.37 g/kg of dry soil,and the average Q_(10)was 1.535,1.585,1.62 and 1.725 for CT,SCR,TYR and CCR,respectively.Increasing maize stalk retention led to the reduction of soil microbial abundance and labile carbon compositions.Pearson correlation analysis showed that soil microbial abundance had a positive correlation with SR,while labile carbon fraction had a negative correlation with Q_(10).In short,increasing the amount of maize stalk retention decreases SR while increasing Q_(10)in northeast China.This research could provide a reference value for balancing carbon sequestration and carbon decomposition in farming practice.