Does conversion of forest to agricultural land change soil carbon and nitrogen? a review of the literature

Soil carbon is a large component of the global carbon cycle and its management can significantly affect the atmospheric CO₂ concentration. An important management issue is the extent of soil carbon (C) release when forest is converted to agricultural land. We reviewed the literature to assess changes in soil C upon conversion of forests to agricultural land. Analyses are confounded by changes in soil bulk density upon land‐use change, with agricultural soils on average having 13% higher bulk density. Consistent with earlier reviews, we found that conversion of forest to cultivated land led to an average loss of approximately 30% of soil C. When we restricted our analysis to studies that had used appropriate corrections for changes in bulk density, soil C loss was 22%. When, from all the studies compiled, we considered only studies reporting both soil C and nitrogen (N), average losses of C and N were 24% and 15%, respectively, hence showing a decrease in the average C : N ratio. The magnitude of these changes in the C : N ratio did not correlate with either C or N changes. When considering the transition from forest to pasture, there was no significant change in either soil C or N, even though reported changes in soil C ranged from ?50% to +160%. Among studies that reported changes in soil N as well as soil C, C : N ratios both increased and decreased, with trends depending on changes in system N. Systems with increasing soil N generally had decreased C : N ratios, whereas systems with decreasing soil N had increased C : N ratios. Our survey confirmed earlier findings that conversion of forest to cropland generally leads to a loss of soil carbon, although the magnitude of change might have been inflated in many studies by the confounding influence of bulk‐density changes. In contrast, conversion of forest to uncultivated grazing land did not, on average, lead to loss of soil carbon, although individual sites may lose or gain soil C, depending on specific circumstances, such as application of fertiliser or retention or removal of plant residues.     

Are there links between responses of soil microbes and ecosystem functioning to elevated CO2, N deposition and warming? A global perspective

In recent years, there has been an increase in research to understand how global changes’ impacts on soil biota translate into altered ecosystem functioning. However, results vary between global change effects, soil taxa, and ecosystem processes studied, and a synthesis of relationships is lacking. Therefore, here we initiate such a synthesis to assess whether the effect size of global change drivers (elevated CO₂, N deposition, and warming) on soil microbial abundance is related with the effect size of these drivers on ecosystem functioning (plant biomass, soil C cycle, and soil N cycle) using meta‐analysis and structural equation modeling. For N deposition and warming, the global change effect size on soil microbes was positively associated with the global change effect size on ecosystem functioning, and these relationships were consistent across taxa and ecosystem processes. However, for elevated CO₂, such links were more taxon and ecosystem process specific. For example, fungal abundance responses to elevated CO₂ were positively correlated with those of plant biomass but negatively with those of the N cycle. Our results go beyond previous assessments of the sensitivity of soil microbes and ecosystem processes to global change, and demonstrate the existence of general links between the responses of soil microbial abundance and ecosystem functioning. Further we identify critical areas for future research, specifically altered precipitation, soil fauna, soil community composition, and litter decomposition, that are need to better quantify the ecosystem consequences of global change impacts on soil biodiversity.     

When do grazers accelerate or decelerate soil carbon and nitrogen cycling in tundra? A test of theory on grazing effects in fertile and infertile habitats

It is generally predicted that grazers enhance soil microbial activity and nutrient availability and promote soil bacteria in fertile ecosystems, but retard microbial activity and nutrient availability and promote soil fungi in infertile ecosystems. We tested these predictions in tundra by comparing grazing effects between fertile and infertile habitats and with/without nutrient manipulation by fertilization. Grazing decreased soil N content in fertile and in fertilized plots in infertile habitats while increased it in infertile tundra habitats, which directly opposed our prediction. We conclude that this unpredicted outcome probably resulted from nutrient transport between habitats. Also contrasting with our hypothesis, grazing increased fungal rather than bacterial abundance in fertilized plots at both habitats. In support with predictions, grazing increased microbial activity for soil C decomposition in fertile but decreased it in infertile habitats. The effect of grazing on soil C decomposition followed same patterns as grazer‐induced changes in the activity of β‐glucosidase, which is an extracellular enzyme synthesized by soil microorganisms for degrading soil cellulose. We suggest that the theoretical framework on grazer–soil interactions should incorporate microbial potential for extracellular enzyme production (‘microscale’ grazer effects) and nutrient translocation by grazers among habitats (‘macroscale’ grazer effects) as important mechanisms by which grazers influence soil processes and nutrient availability for plants at contrasting levels of habitat fertility.