Soil Respiration From Returning Farmland In Small Watershed Scale And Its Response To Soil Temperature And Water

Since soil respiration is the major route for soil C-pool returning CO2 to the atmosphere, small changes in biotic and environment factors may affect soil respiration dynamics, change carbon charactors of ecosystem, and had great influence on global climate.Knowledge of the effects of land-use change on soil respiration under the conversion from cropland to forest and grassland in loess hilly region would be helpful to understand the context of global warming. A field observation was conduct in little water catchment of Yuanzegou, where soil respiration and its relationships with soil water and temperature was measured and analysis under 4 typical land-use types during the growing season. The land-use types include: Cropland(A), Caragana microphylla shrub land(C), fllow grassland(G) and Jujube orchard(J). The main reslts were listed as follows:(1) Land-use change lead to different soil water, temperature, respiration dynamics and their relationships in durinal scales.Soil respirations share same patterns with soil temperature, which mean values followed by C(3.08 μmol CO2·m-2·s-1)>G(2.84 μmol CO2·m-2·s-1)>A(2.45 μmol CO2·m-2·s-1)>J(1.92 μmol CO2·m-2·s-1). J showed the biggest variations of soil respiration(38.1%), while C and G had respiration rates 25.7% and 16.0% higher than A. Delay effects was found between soil respiration and temperature. Relationships between soil respiration and temperature was found significant only in Cropland, while temperature sensitivity(Q10) was lower in other land-use types than cropland. Two factor regression anlysis between soil respiration and temperature, water content showed better result to predict soil respiration among all land-use types..(2)Significant changes of soil water, temperature, respiration dynamics and interactions between them was found among different land-use types after the conversion from cropland. Soil respiration dynamics unimodal change with soil temperature, with peaks appearing at better soil water and temperature conditions among different land-use types. Soil respiration positive correlation with soil water and temperature, while soil water effects was weaken by soil temperature. Two factor liner regression showed better result between soil respiration and soil water content and temperature among different land-use types. Temperature sensitivity of soil repiration changed with soil SOC content, while caused by drought, Q10 of A became higher than j and C had the lowest than other land-use. Mean values of soil respiration rates were list as follow in year 2014 and 2015: A(2.57 μmol CO2·m-2·s-1)>G(2.34 μmol CO2·m-2·s-1)>J(1.49 μmol CO2·m-2·s-1), C(1.80 μmol CO2·m-2·s-1)>G(1.71 μmol CO2·m-2·s-1)>A(1.57 μmol CO2·m-2·s-1)>J(1.42 μmol CO2·m-2·s-,1). Significant drop of soil respiration rates was fund in J conversion from cropland(P<0.05), while contrast trend showed in C. Though no significant difference was found between mean soil respiration rates measuren from fallow land and crop land, huge potential of C output existent due to higher values of Q10 than other land-use. Soil water condition controlled respiration interannual dynimacs. Soil respiration rates measured from A and G droped by 27.7% and 17.4% in 2015 than 2014 effected by drought, while little changes was foun in J.(3)Rain pulse event in summer caused great changes in soil respirations among different land-use types, and soil respiration rates after precipitation was 1.51-3.69, 1.67-4.96 and 0.60-2.08 times greater than pre-rain level among 4 land-use types, with great variations of values and times of respiration peaks. Soil water content showed bilateral control of soil respiration, and A and J was fitted by quadratic functions while C and G was fitted by piecewise funtions. Water threshold was found among different land-use type and listed as follow: C(0.30 m3·m-3)>G(0.20 m3·m-3)>J(0.16 m3·m-3)>A(0.13 m3·m-3). Soil respiration is negative correlation with soil temperature(P<0.05), and showed positive effects when better soil water content level appeared. Hence study conducted by simulation rain may over estimated soil respiration rates during rain event as it failed to simulate soil temperature changes. Under the context of climate change and global warming, soil respiration rates may significant increase due to the changes of precipitation charactors in C and G hilly area of loess plateau.