Dynamics of Soil Nitrogen and Carbon Accumulation for 61 Years After Agricultural Abandonment

Abstract

We used two independent methods to determine the dynamics of soil carbon and nitrogen following abandonment of agricultural fields on a Minnesota sand plain. First, we used a chronosequence of 19 fields abandoned from 1927 to 1982 to infer soil carbon and nitrogen dynamics. Second, we directly observed dynamics of carbon and nitrogen over a 12‐yr period in 1900 permanent plots in these fields. These observed dynamics were used in a differential equation model to predict soil carbon and nitrogen dynamics. The two methods yielded similar results. Resampling the 1900 plots showed that the rates of accumulation of nitrogen and carbon over 12 yr depended on ambient carbon and nitrogen levels in the soil, with rates of accumulation declining at higher carbon and nitrogen levels. A dynamic model fitted to the observed rates of change predicted logistic dynamics for nitrogen and carbon accumulation. On average, agricultural practices resulted in a 75% loss of soil nitrogen and an 89% loss of soil carbon at the time of abandonment. Recovery to 95% of the preagricultural levels is predicted to require 180 yr for nitrogen and 230 yr for carbon. This model accurately predicted the soil carbon, nitrogen, and carbon:nitrogen ratio patterns observed in the chronosequence of old fields, suggesting that the chronosequence may be indicative of actual changes in soil carbon and nitrogen. Our results suggest that the rate of carbon accumulation was controlled by the rate of nitrogen accumulation, which in turn depended on atmospheric nitrogen deposition and symbiotic nitrogen fixation by legumes. Our data support the hypothesis that these abandoned fields initially retain essentially all nitrogen and have a closed nitrogen cycle. Multiple regression suggests that vegetation composition had a significant influence on the rates of accumulation of both nitrogen and carbon; legumes increased these rates, and C3 grasses and forbs decreased them. C4 grasses increased the C:N ratio of the soil organic matter and thereby increased the rate of carbon accumulation, but not nitrogen accumulation.

Anthropogenic ecosystem disturbance and the recovery debt

“…Although nitrogen recovery debts could be expected to be lower than carbon debts because of faster turnover rates of nitrogen1017, our results suggest similar impacts of anthropogenic disturbances in the cycling of both elements. This adds evidence to other large-scale recovery estimations that found similar recovery patterns for the cycling of carbon and nitrogen1819.…”

Landscape Legacies

“…The specific impacts of cultivation vary with soil amendments, such as liming or fertilization, and crop management (Compton et al, 1998;Knops and Tilman, 2000;Richter et al, 2000). The specific impacts of cultivation vary with soil amendments, such as liming or fertilization, and crop management (Compton et al, 1998;Knops and Tilman, 2000;Richter et al, 2000).…”

“…Despite slow rates of accumulation, depleted soil C and N pools recover eventually in most ecosystems (Knops and Tilman, 2000;Hooker and Compton, 2003;Matlack, 2009). The factors that influence C and N recovery operate via their effects on soil development and C and nutrient inputs and include vegetation recovery, climate, and soil texture and mineralogy Ihori et al, 1995;Knops and Tilman, 2000;Post and Kwon, 2000;Springob et al, 2001;McLauchlan, 2006).…”

Dynamics of Soil Organic Nitrogen and its Functions in Soil Carbon Stabilization Processes

“…Although the mechanisms that control long-term N and C cycling are difficult to discern because of complex nature of biological, chemical, and physical interactions among a diversity of microorganisms, organic matter and its functional groups, and mineral surfaces, it has been well known that the stability and accumulation of SON has a positive correlation with the stability and accumulation of SOM (Knops and Tilman 2000). Up to date, however, the dynamics of SON and its function in SOM stabilization has not been well understood because of insufficient knowledge of the composition and structures of SON. In current models of soil C and N cycling, N dynamics are mainly based on external inputs, internal N cycling with the plant-soil system, and N losses (Ogle et al 2007).…”

“…In contrast, SON, which accounts for more than 90% (Vairavamurthy and Wang, 2002), is treated as a black box. Knowledge of SON dynamics and composition is essential for understanding the changing global C cycle because SON is one of the controlling factors in soil carbon storage size and turnover (Knops and Tilman 2000).…”