Interpretation of Soil Carbon and Nitrogen Dynamics in Agricultural and Afforested Soils

part, be attributed to environmental impacts on the controls on SOM dynamics that exist on these sites. It is Interpretation of soil organic C (SOC) dynamics depends heavily impossible to sample the many sites, soil types, manageon analytical methods and management systems studied. Comparison of data from long-term corn (Zea mays)-plot soils in Eastern North ment regimes, landscapes, and climatic regimes in America showed mean residence times (MRTs) of SOC determined enough detail to inform decision-makers on issues such by 14C dating were 176 times those measured with 13C abundance as C sequestration. Methods that incorporate the key following a 30-yr replacement of C3 by C4 plants on the same soils. factors controlling soil C and N dynamics (Fig. 1) need However, MRTs of the two methods were related (r2 0.71). Field to be validated from detailed site measurements. To be 13C MRTs of SOC were also related (R2 0.55 to 0.85) to those useful for answering today’s questions on the potential measured by CO2 evolution and curve fitting during laboratory incufor C sequestration with regard to global change, models bation. The strong relations, but different MRTs, were interpreted that predict pool sizes and turnover rates, and thus overto mean that the three methods sampled different parts of a SOC all C dynamics, must be based on verifiable, analytically continuum. The SOC of all parts of this continuum must be affected derived data. by the same controls on SOC dynamics for this to occur. Methods for site selection, plant biomass, soil sampling and analysis were tested Tracers of C have provided a great deal of information on agricultural, afforested-agriculture, and native forest sites to deteron SOC dynamics (Coleman and Fry, 1991). Carbon mine the controls on SOC dynamics. Soil-C changes after afforestation dating of 14C, occurring in the atmosphere and incorpowere 0.07 to 0.55 Mg C ha 1 yr 1 on deciduous sites and 0.85 to rated into plant components through photosynthesis 0.58 Mg C ha 1 yr 1 under conifers. Soil N changes under afforestation (Campbell et al., 1967; Trumbore et al., 1996), provides ranged from 0.1 to 0.025 Mg N ha 1 yr 1. Ecosystem N accumulation a useful but expensive tracer. Discrimination against was 0.09 to 0.08 Mg N ha 1 yr 1. Soil C and N sequestration but atmospheric CO2 during plant photosynthesis provides not plant biomass were related to soil Ca, Mg, and K contents. Comanother signal (Boutton, 1996). Plants with a C3 pathparative, independent assays of long-term plots provides information way, such as most trees and crops (wheat [Triticum for concept testing and the confidence necessary for decision-makers aestivum L.], soybeans [Glycine max L.], cool season determining C-cycle policies. grasses), result in more negative 13C plant residues than C4 plants such as corn or warm season grasses. This signal can be used to follow SOM dynamics if natural T accumulation and turnover of soil organic succession, or management, has resulted in a plant matter (SOM) is a major factor in soil fertility and switch such as the growth of C4 corn on previously C3 ecosystem functioning and determines whether soils act forest soil or on soil that has grown primarily C3 crops. as sinks or sources of C in the global C cycle (Post and Soil organic matter is comprised of a continuum of Kwon, 2000). Soil organic matter dynamics are complex, materials that range in MRTs of days and months involving a wide array of organic constituents (Sollins through decades, centuries, and millennia. The MRT et al., 1999) with MRTs that range over three orders of of the labeled soil components is dependent both on magnitude (Goh et al., 1989; Paul et al., 2001a). The resistance to decay and extent of protection against deresults are most useful if standardized sampling techcomposition. The length of time that the label has been niques and analyses are combined with available tracers in the soil affects the distribution of the tracer through across a number of long-term experiments or chronodifferent SOM components and strongly influences sequence sites. MRT estimates. Short-term experiments with artificially Determining the impacts of management on soil C added tracers produced in the laboratory (Coleman and and N accumulation is hindered by the small amounts Fry, 1991) and followed in the field for 1 to 2 yr most sequestered annually relative to the large pools in the often yield results with MRTs of months to a few years soil as well as the considerable spatial heterogeneity regardless of tracer employed. The 13C plant-switch of SOM (Morris, 1999; Stoyan et al., 2000; Mulla and methods often use experiments that have been in place McBratney, 2000). This heterogeneity, can in a large for decades. They produce results in this time frame (Six and Jastrow, 2002). Carbon dating shows much E.A. Paul, J. Six, and K. Paustian, Natural Resource Ecology Lab., greater ages with some SOC being thousands of years Colorado State Univ., Ft. Collins, CO 80523; S.J. Morris, Dep. of Biology, Bradley Univ., Peoria, IL 61625; E.G. Gregorich, Eastern old (Paul et al., 2001b). Cereal and Oilseed Research Centre, Agriculture Canada, Ottawa, The dynamics of SOC can also be measured by soil ON KJA OC6, Canada; J. Six, Present Address: Dep. of Agronomy incubation in the laboratory to allow biological mineraland Range Science, Univ. of California, Davis CA 95616. Received ization of C. Curve analysis of CO2 evolution rates, over 11 Feb. 2002. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 67:1620–1628 (2003).  Soil Science Society of America Abbreviations: MRT, mean residence time; SOC, soil organic C; SOM, soil organic matter. 677 S. Segoe Rd., Madison, WI 53711 USA 1620 Published September, 2003