A recent study done in the Loess Plateau, China, looked at the effects of abandoned farm soil at several depths regarding soil organic carbon (SOC) and total nitrogen (TN) levels over a period of thirty years. The study determined that the older the abandoned area and the greater the depth, the higher the pre-farming carbon and nitrogen levels present, which correlate to a better recovery rate of the entire ecosystem in an yet unknown manner.
According to another soil study by Wang et al, both SOC and TN concentrations increase in a positive linear relationship, simply stated, soil carbon and total nitrogen in soil increase at the same rate when both are present. The Loess Plateau came to the same conclusion from their data analysis.
Specified areas including several control sites studied sampled depths of 0-5cm, 5-10cm, 10-20cm, 20-30cm, 30-50cm, 50-70cm, and 70-100cm. The specified depth material was mixed into one sample and a 2mm screen was used to obtain the final testing material.
The study took a close look at deeper levels of soil, therefore, I will start with the deepest depths and work to the surface. SOC levels at 70-100cm showed no noticeable increases, yet there was a general trend that carbon levels were lower in younger fields and higher in older fields. TN levels showed the same patterns as SOC at 70-100cm, which is based on the positive relationships previously mentioned.
At depths of 30-70cm TN decreased significantly, then increased to previous levels before farming. After thirty years, the 20-70cm depths were back to normal carbon levels prior to farming activities.
At depths of 10-30cm there was no noticeable changes in total nitrogen. What the scientists found interesting was that SOC levels did not increase significantly in the 5-20cm depths.
Levels of total nitrogen at 0-5cm were similar to SOC, they were much higher in areas following abandonment. What was interesting was that for the first 13 years TN levels were about the same and for the first 20 years SOC levels were the same, then there was a large increase afterwards. Consistent farming seems to have a significant effect on the 0-5cm soil levels based on the recovery times of the top layer.
Based on the above data, there were several factors found that explained why these patterns were occurring. The first is that initial reduction in crop residue (plant matter) from harvesting crops reduced soil regeneration.
The second factor is that belowground biomass (plants and animals below ground) which include; dead roots, mycorrhizae, and exudates, are important to recharging carbon in the soil, which leads to higher TN levels. Additionally the disturbance of soil through farming lowered the amount of biomass available year to year.
The third factor to explain why the SOC and TN levels in the field were initially low and then increased to previous levels comes from the availability of plant matter. The initial loss of decomposition from lost plant matter had a large impact on available carbon and nitrogen for several years. This coupled with greater and greater amounts of plants helped to regenerate the concentrations of carbon and nitrogen over time.
The information above clearly shows that further studies of soil depths are needed to improve ecological conditions. One must look past the initial soil layer, and find out which plants, animals, and processes are best to reduce the amount of time a field takes to replenish its SOC and TN concentrations.
Smt8 (Stephen Truch)
Picture courtesy of http://www.upload.wikimedia.org
Source: Deng, Lei; Shangguan, Zhou-Ping; Sweeney, Sandra. “Changes in Soil Carbon and Nitrogen following Land Abandonment of Farmland on the Loess Plateau, China.” (August, 2013): 8p. PLOS ONE. 14 October. 2013.