Maintenance Requirements from Incomplete Budgets

In a carbon constrained economy, the ability to accurately predict the impact of agricultural management, climate, and landscape variability on soil carbon rate constants is needed. A number of different approaches have been used to define rate constant. Some approaches define SOC as a single pool while other approaches separate SOC into multiple pools (Coleman and Jenkinson 1996; Clay et al. 2006). Unless a common approach for defining the rate constants is used they cannot be directly compared. Once the rate constants are defined, the impact of a management change can be calculated directly from the data set.

Defining SOC and non-harvested carbon (NHC) mineralization rate constants requires accurate measures of organic carbon inputs, outputs, and a clear mechanistic understanding of the C turnover processes. Obtaining good measures of above-ground biomass is relatively easy and is typically accomplished by weighting the amount of biomass returned or estimating the value from the harvest index. However, obtaining accurate measures of below-ground biomass is very difficult (Kuzyakov and Domanski 2000; Amos and Walters 2006). In the past, nearly all efforts have underestimated below-ground biomass because they do a poor job at measuring small roots, root exudates, and below-ground-biomass-derived CO2. The ability to predict the consequences of agricultural intensification on long-term soil sustainability is limited by the quality of available data. To maintain SOC it has been estimated that between 2,500 kg biomass C ha-1 in Iowa (Larson et al. 1972) and 5,600 kg C ha-1 in Minnesota are required (Huggins et al. 1998). These differences may be real or artifacts from the approaches used to estimate the maintenance requirements.

One of the most difficult input values to measure is below-ground biomass. Below-ground biomass consists of the carbon contained in the root biomass, exudated carbon, and respired CO2 by soil microorganism. Several recent reviews of below-ground biomass are available (Ehleringer et al. 2000; Amos and Walters 2006; Bolinder et al. 2007). These reviews show that extreme variability in root to shoot ratios exists and that all sources of carbon should be included in budget calculations. Different efforts have calculated below-ground biomass differently. For example, Larson et al. (1972) did not include below-ground residues (roots) in their non-harvested biomass calculations, while Huggins et al. (1998) considered below-ground biomass in their calculation. The amounts of roots included in the calculated value impact the maintenance value and the resulting removal estimate.

Field maintenance rate calculations (the NHC level required to maintain SOC levels) are further complicated by above- and below-ground biomass that often have different mineralization rates. Gale and Cambardella (2000) reported that in no-tillage, 75% of the new C incorporated into SOC was root-derived, while a large percentage of surface residue was released as CO2. Barber and Martin (1976) had similar results and reported that 50% of the root-derived C was retained in SOC while only 13% of shoot-derived C was retained in SOC. Increased retention of root C could be attributed to a variety of factors including: (i) greater biochemical recalcitrance of root biomass; (ii) physical protection of root biomass within aggregates from degradation; (iii) large but unknown amount of exudates and fine roots providing carbon into below-ground systems; (iv) individual carbon sources that may interact to influence net carbon mineralization (Puget and Drinkwater 2001); and (v) lower O2 concentrations with increasing soil depth that can result in reduced root decomposition. It has been hypothesized that the mineralization of exudates and fine roots may slow down the mineralization of relic SOC and larger roots. Puget and Drinkwater (2001) concluded that shoot residues are broken down quickly, whereas the slow decomposition of root litter is responsible for short-term structural improvements resulting from green manure crops. To predict the consequences of management on SOC, the mineralization kinetics and total amounts of C contained in the different C sources must be known.

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