Soil Carbon Change Due To Intensive Farming Practices In Northern China

Intensive agriculture helps to produce more food from limited land resources to feed ever increasing population of the world. Vegetable production in a greenhouse is popular worldwide, especially in China as one of the world’s greatest economies. Wheat-maize double cropping system is also important to secure grain supply. The farming practices associated with these intensive agricultural systems have profound impacts on the dynamics of soil carbon. There is need to evaluate these systems for their potential for carbon sequestration in soil to mitigate greenhouse emissions on one hand and improve the sustainability of these systems on the other hand. Soil carbon is composed of two components, i.e., soil organic carbon (SOC) and soil inorganic carbon (SIC). Although there has been many studies to understand SOC dynamics, however very few studies have focused on the SIC dynamics in these systems.We selected two greenhouse vegetable production systems in Shouguang, Shandong province and Quzhou, Hebei province. A field cropping system (wheat-maize) was also evaluated. The greenhouse vegetable production systems at Shouguang focused on the two types of irrigation and three types of organic amendments while the greenhouse at Quzhou had conventional, organic and integrated farming practices. Similar treatments were also evaluated in open field wheat-maize cropping systems and a natural fallow plot was used as reference at the Quzhou. The SOC and SIC contents and stocks were measured down to 200 cm depth; SOC in soil particle size fractions was also measured to evaluate the OC with various stages of stability.δ13C and δ15N were also measured for the bulk and soil fractions which enabled us to evaluate the changes of SOC and SIC in these systems in details. Additionally δ13C values were used to calculate the percentage of pedogenic carbon (PC) and lithogenic carbon (LC) in the total SIC.The greenhouse at Shouguang consisted two main irrigation treatments, drip irrigation with reduced optimized fertigation (D) and furrow irrigation with conventional excessive fertilization (F). There were three organic amendments manure only (M), manure plus wheat straw (MWS) and manure plus Corn straw (MCS). The results showed that SOC stocks in drip irrigated plot were significantly increased with the application of organic amendments down to 200 cm, which were in the order of:MWS> MCS>M. In furrow irrigated plot SOC stocks also followed same order but only FMWS had significantly higher stocks than other two organic amendment treatments at 0-100 cm. Significantly higher OC stocks were stored in drip irrigated plots with optimum mineral fertilizers compared to furrow plot with excessive mineral fertilization. Drip irrigated treatment accumulated 3.98 and 1.53 Mg SOC ha-1 yr-1 at 0-100 and 0-200 cm respectively than furrow irrigated plot. The particle size fractionation revealed that this increase in the OC was more stable as this was associated with the silt and clay size particles fractions. The δ15N isotopic evaluation of bulk soils and fractions also confirmed that generally the SOC in drip irrigated plot had more relative stability compared to furrow irrigated plots. Furthermore δ13C and δ15N isotopic evaluation of entire soil profile revealed that this increase of SOC in drip irrigated plots did not came due to slowdown effect of decomposition rates but it was due to the addition of new carbon materials to soils which were probably supplied by the increased root, macro and microbial biomass.The SIC measurement revealed that drip irrigated plot had 55.5 Mg ha-1 more SIC than flood irrigated plot. Partitioning of the SIC pool in to PC and LC stocks showed increase of 0.3 Mg PC ha-1 yr-1 compared to conventional furrow irrigated plot. Furrow irrigated plot showed the loss of 7.9 Mg LC ha-1 yr-1 to the depth of 0-200 cm over the seven years of experimental period. The reduction in the LC was due to dissolution of carbonates in the furrow irrigated plot which was probably triggered by the decrease in soil pH due to excessive use of mineral fertilizers and irrigation application. The straw addition showed a positive impact on further reducing the carbonate dissolution and promoting neo-formation of pedogenic carbonates. Therefore, drip irrigation along with straw application has the potential for improving the sustainability of greenhouse vegetable production system.Quzhou greenhouse vegetable production focused on three fertilization treatments:Conventional fertilization with approximately 25% nutrients supplied from organic fertilizer and 75% from mineral fertilizers (CON); Integrated fertilization that supplied 50% of nutrients from organic and 50% from mineral fertilization this treatment was named as (LOW); and the organic treatment where all the nutrients were supplied from the organic fertilizers (ORG); Natural fallow plot without any management practice (NF) was used as a reference. The results showed that ORG and LOW treatments increased OC contents and stocks not only at the surface soil layers but also down to 200 cm soil depth. The δ15N values in soil profile suggested that this SOC stored below 100 cm was highly sensitive to recent change in fertilizer managements. Compared to NF,35.8 and 28.6 Mg SOC ha-1 in ORG and 28 and 2 Mg SOC ha-1 in LOW treatments were increased, whereas CON lost 22.5 and 13.6 Mg SOC ha-1 at 0-100 and 0-200 cm depth, respectively. Considering 0-200 cm depth ORG treatment increased SOC stocks to around 2 times compared with CON treatment with the annual increase rate of 5.3 Mg C ha-1 yr-1. Particle size fractionation revealed that although the ORG and LOW treatments had highest OC contents in labile coarse sand size particles compared to other treatments but they continued to increase their OC transfer to non-labile particles. At 0-20 cm depth all the greenhouse treatments had more than 80% of their total SOC stored in silt plus clay size particles compared to 74% in NF. However at 20-40 cm depth, NF had higher SOC stored in silt plus clay particles with 85% followed by ORG with its share of 75%. This is attributable to very fast turnover rates of SOC in ORG treatment due to which most of its OC ended in silt and clay size particles. This stable OC in ORG treatment can play important role for the sequestration of atmospheric CO2.Compared to NF, the SIC stocks were increased by 44,40 and 36 Mg SIC ha-1 at 0-200 cm depth for CON, LOW, and ORG, respectively. The division of SIC into PC and LC revealed that significantly highest PC stocks were in CON followed by the LOW treatments; however, this increase came at the expense of significant loss of LC stocks. Soil samples collected at 2002 provided the evidence that the PC in CON and LOW treatment were of pedo-lithogenic origin and this had no C sequestration potential while the PC in ORG were of pedo-atmogenic origin having real potential for C sequestration. The positive correlation between SOC and PC in ORG and LOW treatments further validated that there was a close linkage between SOC and PC formation.The open field wheat-maize experiment showed that over the 39-year period, organic fertilizers at high and low rate (OFH and OFL), mineral fertilizers (MF), and a control site without fertilizers (CK) showed an increase of PC compared to a natural fallow plots (F). The main pathway of SIC accumulation was the neoformation of pedo-atmogenic carbonates contributing to C sequestration of at least 0.38,0.27,0.23,0.12 Mg C ha-1 yr-1 for the OFH, OFL, MF, and CK treatments, respectively. The LC stock remained similar in all treatments except for the CK, where LC was significantly lower than all of the other treatments which suggested dissolution. An increase in OC stocks in response to organic fertilization was not limited to the surface soil, but it continued down the soil profile to a depth of 160 cm. The maximum potential for neoformation of PC depends on Ca2+ and Mg2+ availability; in this study these cations were provided by irrigation water. However, organic and mineral fertilizers modify this potential. Without organic and mineral fertilization, the PC formed at the expense of dissolution and re-precipitation of LC, even when substantial quantities of Ca2+ and Mg2+ were present in the soilOur research concluded that drip irrigation with optimized fertigation is a potential option to improve the sustainability of the greenhouse vegetable production systems. This will not only save the energy on the additional conventional furrow irrigation but also have the potential of C sequestration by increasing organic and inorganic carbon storage in soils which have higher stability. Organic farming is also a viable option to further improve the sustainability of these systems. Therefore the combination of organic or integrated fertilization with drip irrigation will be a good option for improving the sustainable greenhouse vegetable production that can increase production yields and carbon storage in soils. Furthermore, our experimental results indicate that the neoformation of PC should be considered during estimation of soil carbon stocks and sequestration for the development of optimal fertilization, irrigation and land use practices.