Tea time for soils : decomposition experiments in Swedish long-term field trials

Abstract: Abstract Soils represent a large carbon pool, with almost twice the amount contained in living plant biomass and the atmosphere combined. Consequently, soil has a significant impact on the global C cycle and it is suggested that soil organic C (SOC) sequestration is one of the most cost-effective alternatives to counteract climate change. The literature was reviewed regarding the influence of abiotic and biotic factors on SOC dynamics, and how SOC stocks vary with management practices and cropping systems. I also screened the literature for different meth-odologies to study carbon and litter decomposition. I decided to use the Tea bag index (TBI) approach. The TBI uses two types of commercially available tea bags, and characterizes decomposition dynamics in terms of two parameters, the decomposition rate (k) and stabilization factor (S). I hypothesized that the TBI would be sensitive enough to quantify the impact of common agricultural practices on litter decomposition under a wide range of pedo-climatic conditions. I selected treatments from 13 long-term field trials that allowed me to compare: perennial forage versus annual crops (N = 4), fertilized versus unfertilized fields (N = 6) and different tillage practices (N = 3). The results showed that the two TBI parameters were sensitive to both management practices and cropping systems. Fertilized plots showed higher stabilization (S) than unfertilized plots, but there were no differences on decomposition rates (k). The effect of different tillage practices on k and S were variable across sites and treatments, although ploughing tended to result in lower decomposition rates, compared to direct drilling or shallow cultivation treatments. The TBI was most sensitive when comparing perennial forage versus annual cropping systems, where the measurements for forage generally indicated higher S values and lower k values. The results also showed that climate, through precipitation and air temperature, had a large impact on stabilization (R2 = 0.33), while its effect on decomposition rates was limited. The decomposition rates were found to be significantly affected by soil properties such as clay content (R2 = 0.13) and soil C/N-ratios (R2 = 0.19). It can be concluded that the TBI approach is a useful tool to characterize decompo-sition dynamics and to identify climate-smart agricultural management practicies under different pedo-climatic conditions.