A Model of the Physical Protection of Organic Matter in Soils

Abstract

Current computer simulation models do not treat the physical protection of organic matter in soil mechanistically. A model is presented that describes physical protection explicitly as a function of the capacity of clay particles and aggregates to hold organic matter. The net rate of decomposition of organic matter depends not simply on soil texture but on the degree to which the protective capacity of the soil is already occupied. The rate at which organic matter becomes protected depends on both the amount of free organic matter and the degree to which the protective capacity is filled. The rate at which organic matter is released from protection depends only on the amount of protected organic matter. The model closely followed the buildup and decline of organic matter in 10 soils to which grass residues were added each year for a period of 10 yr and then left without addition for a further 10 yr. An estimate made with the model of the maximum capacity of each of these soils to protect organic matter was closely correlated with the clay fraction in the soils. With this model, we were better able to predict the buildup and decline in amounts of soil organic matter in soils of different textures and initial organic matter contents than with conventional, implicit descriptions of protection. Our model accounted for 80% of the variance in the experimental data compared with 77 and 75% using the other two models. Secondly and importantly, the capacity of soil to protect organic matter was found to be positively and well related with the clay content of the soil in our model, whereas similar relationships were not found between soil texture and the parameters controlling protection in the other models.

Soil carbon, after 3 years, under short-rotation woody crops grown under varying nutrient and water availability

“…They were not suggesting that all soils had a minimum of 21.1 g C kg À1 soil but that they could maintain that amount through physical stabilization. The soils in our study were generally below 10 g C kg À1 soil at all depths so, according to the model developed by Hassink and Whitmore [38], there is potential to increase the soil C contents of these soils. This was not observed in our study, where there was a loss of C by the third year as compared with pre-harvest.…”

“…The physical capacity of a soil to protect organic matter is dependent on soil texture and the amount of organic matter already protected (i.e., stabilized) [18,32,37,38]. Hassink and Whitmore [38] developed a model relating the physical protection of organic matter to soil texture.…”

“…Hassink and Whitmore [38] developed a model relating the physical protection of organic matter to soil texture. Their model suggested that soils, regardless of texture, could maintain a minimum of 21.1 g C kg À1 soil.…”

“…However, the proportion of C stabilized in the soil for the different samplings remained remarkably consistent ( Table 2). The C stabilization model of Hassink and Whitmore [38] described a steady-state condition, where the amount of labile C entering the stabilized C pool was equal to the amount of stabilized C entering the labile C pool. This condition seems to describe our observations where the proportion of stabilized C remained approximately in the mid-to upper 60 percentile.…”

Clay-organic complexes in a Polish loess soil

“…This low density of complexes provides an explanation for the observed decrease in soil density with increasing concentration of OM. The complexing stabilizes the clay and thereby gives it a degree of protection against dispersion in water Emerson et al, 1986;Hassink and Whitmore, 1997). Dexter et al (2008) showed that, whereas non-complexed clay disperses readily in water, the complexed clay does not.…”