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Controls on Soil Organic Matter Dynamics. A Cross Site Comparison

Author

Eldor A. Paul,
Colorado State University

Executive Summary 
The dynamics of soil organic matter (SOM) is a major factor in soil fertility and determines whether soils sequester or lose carbon under different management practices.



The dynamics of soil organic matter (SOM) is a major factor in soil fertility and determines whether soils sequester or lose carbon under different management practices. SOM dynamics involve a wide array of organic constituents with mean residence times (MRTs) that range from months to a few years for recently added residues and microbial products that constitute the active fraction. The slow fraction, stabilized by aggregation and comprised of such constituents as the particulate organic matter, often persists for decades. The resistant fraction comprised of non-hydrolysable soil humics and materials stabilized by organo-mineral interactions persist for hundreds to thousands of years. These fractions are studied by methods such as the separation of particulate organic matter, aggregate analysis acid hydrolysis and laboratory incubation. These are most useful if standardized sampling techniques and analyses are combined with available tracers such as 14C, 13C and 15N on long-term experiments or chronosequence sites that represent the wide diversity of landscapes in North America.  

     
It is impossible to sample the many sites, soil types, management regimes, landscapes and climatic regimes in enough detail to inform decision-makers on issues such as C sequestration. Methods that incorporate the key factors controlling soil C and N dynamics (Figure 1) need to be validated from detailed site measurements. These need to be combined with models to allow for extrapolation to other sites and to broader landscapes. To be useful for answering today's questions on the potential for C sequestration with regard to global change, models that predict pool sizes and turnover rates, and thus overall C dynamics, must be based on verifiable, analytically derived data.  

     
A cross-site comparison from Eastern North America involves an analysis of data in a cooperative study between Colorado State University, Michigan State University and Bradley University in Illinois and Agriculture Canada in Ottawa. The Western study involves site in Colorado, Nebraska and Western Canada in a study that involves Canadian scientists, USDA and Colorado State University. Comparison of data from long-term corn-plot soils, with both a 13C and 14C naturally- occurring label in Eastern North America showed mean residence times (MRTs) of soil organic carbon (SOC) determined by 14C dating were 176 times those of field 13C MTRs measured with abundance experiments following a 30 yr replacement of C3 by C4 plants on the same soils. However, MRTs of the two methods were closely related (r2= 0.71). The field 13C MRTs of SOC were also related (R2 =0.55 to 0.85) to those measured by 13CO2 evolution and curve fitting during laboratory incubation. The strong relations, but different MRTs, were interpreted to mean that the three methods sampled different parts of a SOC continuum. The SOC of all parts of this continuum must be affected by the same controls on SOC dynamics for this to occur.  

     
Forested soils present major analytical problems in determining SOM dynamics and estimating rates of soil C and N change with land use conversions from agriculture to forest. There are rarely replicated long-term experiments of the effects of land use change or management. Trees growing in Eastern N. America on afforested sites that were in mixed cropping regimes during their cultivation period do not provide a useful 13C signals for estimating turnover rates. Thus, the dominant methodology is a ‘paired-plot’ design, with identification of adjacent fields, on the same soil type, for comparative, chronosequence studies. For such studies, selection of the areas to be compared and the method of sampling are important issues.  

     
Methods for site selection, plant biomass, soil sampling and analysis were tested in a comparison of agricultural, afforested (deciduous and conifer) and native-deciduous forest sites to determine the controls on SOM dynamics in the more humid conditions of Eastern North America. Soil-C changes after afforestation were -0.07 to 0.55 Mg C ha-1 yr-1 on deciduous sites that included oaks and maples and -0.85 to 0.56 Mg C ha-1 yr-1 under conifers (pines). Soil N changes under afforestation ranged from -0.1 to 0.025 Mg N ha-1 yr-1. Ecosystem N accumulation was –0.09 to 0.08 Mg N ha-1 yr-1 and could not be accounted for by known levels of N2 fixation or atmospheric inputs. Soil C and N sequestration but not plant biomass was related to soil Ca, Mg and K contents. High levels of nutrient especially Ca resulted in C and N sequestration in pines but low levels were related to losses.  

     
Further studies on cross-site comparisons of tracer and long-term management data will involve the incorporation of data from grassland and dry-land cultivated plots of short-grass prairie region in Nebraska and Colorado as well as the further analysis of data from the mixed-grass region of Western Canada.  

     
References  

     
Morris, S.J. and E.A. Paul. 2002. Forest ecology and soil organic matter. In: J.Kimble, R. Birdsey, R. Lal, R. Follett and J. Heath (eds.). The potential of US forest soil to sequester carbon and mitigate the greenhouse effect. CRC Press Boca Rotan Fl.  

     
Collins, H.P., E.T. Elliott, K. Paustian, L.E. Bundy, W.A. Dick, D.R. Huggins, A.J.M. Smucker, and E.A. Paul. 2000. Soil Carbon pools and fluxes in long-term Corn Belt agroecosystems. Soil Biol. & Biochem. 32:157-168.  

     
Paul, E.A., H.P. Collins, and S.W. Leavitt. 2001. Dynamics of resistant soil carbon of Midwestern agricultural soils measured by naturally-occurring 14C abundance. Geoderma 104:239-256.  

     
Follett, R.F., E.A. Paul, S.W. Leavitt, A.D. Halvorson, D. Lyon, and G.A. Peterson. 1997. Carbon isotope ratios of Great Plains soils in wheat-fallow systems. Soil Sci. Soc. Amer. J. 61:1068-1077.  

     
Martel, Y.A. and E.A. Paul. 1974. Effects of cultivation on the organic matter of grassland soils as determined by the fractionation and radiocarbon dating. Can. J. Soil Sci. 54:419-426.  

     
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Figure 1. Process controls of soil organic matter dynamics
     

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