Changes in soil carbon and nitrogen stability and aggregation across urban, forest, and agricultural ecosystems

Background/Question/Methods: While forests are known to retain a high percentage of added nitrogen (from 30-80%), it is also important to investigate urban and agricultural systems that are known sources of nitrogen (N) pollution. Long-term N retention is affected by the stability of N stored in soils. Under changing conditions, labile N may become remineralized, potentially impacting water and air quality, as well as plant growth.  In contrast, stable soil N may be a sink that mitigates N pollution on decadal time scales. The main objective of this study was to quantify stable and labile carbon (C) and N pool sizes in the soil organic matter of forest, urban, and agricultural ecosystems. A stratified-random design was utilized in which 4 replicate sites of each of the three most common land-use types in Centre County, Pennsylvania were sampled. Selected sites were located on lands owned by the Pennsylvania State University and represent sources (agricultural and urban) as well as sinks (forest) for anthropogenic N in the region. The top 15 cm of soil were sampled for this study. Using a biological fractionation approach, long-term laboratory incubations were leached repeatedly in order to separate the total soil N pool into labile and stable pools. Using a physical fractionation approach, we quantified the C and N in microaggregates that are expected to have long mean C and N residence times compared with other soil physical fractions.

Results/Conclusions: Soil total N concentrations were highest in the urban sites (0.27%), followed by forest sites (0.22%), with agricultural sites (0.14-0.20% for conventional till and no-till, respectively) having the lowest concentrations of soil N. Soil total C concentrations were highest in forest (3.11%), intermediate in urban (2.96%), and lowest in agricultural sites (1.26-1.97%). The labile C pool determined from CO₂ measurements of the incubated soils was greatest in urban ecosystems (2394 mg C/kg soil), least in agricultural ecosystems (850-1476 mg C/kg soil), and intermediate in forest ecosystems (1957 mg C/kg soil). While total microaggregation, making up 45% of bulk soils across ecosystems, did not vary by land-use type, the location of microaggregates did vary. Urban and no-till agricultural sites were found to contain more microaggregates within macroaggregates, while conventional till sites contained more N in free microaggregates. Results suggest that the stability of soil C and N varies among land-use types, and that the variation is correlated with changes in the types of microaggregates present in soils.