Effects of straw return and regional factors on spatio-temporal variability of soil organic matter in a high-yielding area of northern China
Introduction
There is an enormous carbon (C) reserve in soil in the form of soil organic matter (SOM) that stores at least three times as much C as either the atmosphere or living plants (Lal, 2004, Schmidt et al., 2011), and is a significant part of the global terrestrial C pool (Mariam et al., 2007). Maintenance of SOM content provides an important opportunity for mitigation of greenhouse gas emissions by C sequestration. Increasing SOM content can also have positive effects on nutrient release and immobilization in soils (Yu et al., 2006), regulating soil porosity and structure (John et al., 2005), increasing soil permeability and reducing erosion (Carter, 2002), and improving soil biological activity (Liu et al., 2010). Agricultural practices to increase SOM contents include no-till farming with crop residue retention, incorporation of forages in the rotation, maintenance of positive nutrient balance, application of manure and other biosolids, conversion of agriculturally marginal soils to perennial land use, and restoration of degraded soils and wetlands (Whitbread et al., 2003, Su et al., 2006, Lal, 2007).
China's arable areas account for 7% of the world's total cultivated land and play an important role in global C sequestration (Yan et al., 2007). The North China Plain (NCP) has about 18 million ha of farmland and is China’s main grain production area contributing about 30% of total national food production. The region has an intensive cropping system with winter wheat and summer maize every year. Some studies have investigated the long-term effects of irrigation, fertilization, tillage and residue management on the SOM accumulation process in the NCP. Liu et al. (2010) found that returning straw and adding farmyard manure can significantly increase SOM levels compared with the application of chemical fertilizer only. Results from long-term experiments at five research stations across the NCP, at Quzhou, Wuqiao, Dongbeiwang, Luancheng, and Hengshui, showed that agricultural practices such as optimal irrigation and fertilization, larger fertilization rates, residue return and rotary tillage have had significant impacts on SOM storage over the last two to three decades (Zhao et al., 2013a). In this regard, the application of the EPIC model (Williams, 1995) to data from the experimental stations indicated that optimal irrigation and fertilization, reduced tillage and residue return were all effective strategies for improving SOM content and storage in the NCP (Zhao et al., 2013b). However, these studies were conducted at field scale and the results are generally site specific. Regional factors affecting SOM accumulation, such as soil type, soil texture topography, and land use conversion, all vary at different experimental sites (Mclauchlan, 2006, Kong et al., 2006, Wang et al., 2012), which are not fully described by EPIC or other models. In addition, some studies found that long-term mineral fertilizer inputs may promote or impede SOM accumulation (Yang et al., 2003, Gong et al., 2009, Niu et al., 2011). Therefore previous plot-scale studies have limited value for determining the impacts of agricultural management on SOM of the NCP at the regional scale. Determination of the spatial variation of SOM at regional scale, and its changes over time, are important for estimating changes in soil C sequestration in the main grain production areas of China.
Huantai County is a typical high-yielding area in the NCP. Agriculture in this county is dominated by a rotation of winter wheat and summer maize that accounts for 87% of all crops planted in recent decades (Liu et al., 2005), and there have been distinct changes to straw management practices over the last two to three decades. From the early 1980s to the middle 1990s high grain production relied on sufficient to excessive amounts of fertilizer applications (Liu et al., 2005). Most of the straw produced was burnt in the field or used as domestic fuel by local farmers (Zhang et al., 2008). This led to atmospheric pollution, greenhouse gas emissions (CO2) and soil nutrient depletion when straw was removed from farmland. Under this production pattern 14,218 kg CO2 ha−1 was emitted yearly (Shi, 2013). With the promotion of the use of combine harvesters and field straw choppers around the mid-1990s straw management was transformed within the county leading to crop residue retention and the improvement of soil fertility (Zhang et al., 2008). However, the impacts of the changes in practices on SOM accumulation not yet been quantified.
The objectives of this study are to determine the changes of SOM in space and time when significant changes in straw retention were introduced, and to explore the effects of straw retention and regional factors on spatio-temporal variation of SOM. The results should lead to the promotion of practices that improve soil fertility and to improve understanding of C sequestration in the NCP.
Section snippets
Study area
The study was conducted in Huantai County in the southeastern NCP, downstream of the Yellow River, on the south bank of the Xiaoqing River, and administratively attached to Zibo city in Shandong Province. The county is located at 36°51′50″–37°06′00″N and 117°50′00″–118°10′40″E, with an area of 509.5 km2 (Fig. 1). The study area covers a transient zone from the alluvial plain in central Shandong to the Yellow River plain in the north. The topography slopes slightly from the southwest to northeast
Analysis of variance of soil organic matter
The results of the K–S test (p > 0.05) showed that SOM data Huantai County was normally distributed at each of the three sampling times (Table 1). The SOM data at three sampling times had a middle degree of variability (Hu et al., 2007), shown by the relatively small coefficients of variation (CV). Mean values for SOM varied between 13.24 ± 2.32, 15.06 ± 1.93 and 18.73 ± 4.06 g kg−1 (mean and standard deviation) in 1982, 1996 and 2011, respectively. SOM increased significantly (p < 0.05) over time, but the
Conclusions
The mean SOM contents in the high-yielding area of Huantai County were 13.24, 15.06 and 18.73 g kg−1 in 1982, 1996 and 2011, respectively. The rate of increase in SOM from 1996 to 2011 was as much as twice that during 1982–1996. These results indicated that the effects of human activities on SOM content were greater during the period from 1982 to 2011. During 1982–1996 increases of SOM were due to manure and chemical fertilizer inputs caused by adjustments of land use type and changes in
Acknowledgements
The study was funded by the National Key Technology R&D Program of China (2012BAD15B01) and the Program on Innovative Research Talent and Team, China Ministry of Agriculture (2012022). We thank Dr. Ian Willett and Dr. Nancy Pontius for providing help in revisions of this manuscript.
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