RESIDUE DECOMPOSITION SUBMODEL1

S.J. van Donk, J.L. Steiner, H.H. Schomberg, and P.W. Unger

Model Overview

This submodel simulates the decrease in crop residue biomass due to microbial activity. The decomposition process is modeled as a first order reaction with temperature and moisture as driving variables. Decomposition is a function of decomposition days; under optimum temperature and moisture conditions one decomposition day per day is accumulated. Only a fraction of a decomposition day is accumulated if conditions are less than optimum. Biomass remaining after harvest is partitioned between standing, flat, buried, and root pools. Below ground biomass (buried and root) decomposition is calculated for each soil layer.

Residue from different crops may decompose at different rates. Since residue decomposition can require a long period of time, crop residue biomass from sequential harvests is accounted for in three separate pools. Biomass from the most recently harvested crop will be in pool one, biomass from the penultimate crop in pool two, and there is a third pool for biomass from the oldest crop(s). After harvest, any residue biomass remaining from a previous crop is moved into the older age pools and residue from the just harvested crop is moved into pool one.

Standing residue losses not only result from microbial activity, but also from physical forces. Physical transfer of crop residue from the standing biomass pool will reduce both the stem population and standing biomass. A daily estimate of the standing stem population is required in order to evaluate the vertical stem area that the wind encounters. This area is quantified by the stem area index, which is calculated from standing stem number, stem height and stem diameter. It affects aerodynamic resistance and, ultimately, wind erosion. Stems start to fall over after a threshold of cumulative decomposition days since harvest has been reached. Stem area index decreases proportionally with decreasing standing stem number.

Both standing and flat crop residue provide cover to the soil surface, protecting it against wind erosion. Soil cover from standing residue is typically small. It is calculated from stem number and stem diameter. Soil cover from flat residue is calculated from flat residue mass. Tillage may alter the amount of residue in the different pools.

Decomposition

The general decomposition equation is a simple first order rate loss equation (Steiner et al., 1999):

where Mt is present biomass (kg m-2) in the standing, flat, buried, or root pools; Mo is initial biomass (kg m-2); k is a crop specific rate constant (kg kg-1 day-1); and CDD is cumulative decomposition days (day): a weighted-time variable calculated from functions of temperature and moisture. WEPS uses the same k-value for the standing, flat, buried, and root residue pools. Examples of slowly decomposing residue are corn and sorghum (k = 0.01 kg kg-1 day-
1). Alfalfa residue decomposes much faster (k = 0.05 kg kg-1 day-1, Figure 1).

For most crops there are only stems left after harvest. However, there may be situations where residue includes leaves and reproductive material (e.g. chaff, awns) as well. Stems decompose at a rate k, leaves at a rate 3k, and reproductive material at a rate 1.5k. The pool with the oldest residue (pool three) may include residue from more than one crop if decomposition is very slow. The k value for this pool will be that of the 'newest' of these old crops.

Optimum moisture and temperature conditions result in the accumulation of 1 decomposition day for each day of the simulation. When moisture or temperature limit the rate of decomposition, the minimum of the moisture and temperature functions is used to accumulate a fraction of a decomposition day.

Biomass loss is calculated using the numeric form of equation 1.1 as follows:

where Mt-1 is biomass (kg m-2) on the previous day and DD is decomposition day:

where TF is a temperature function and MF is a moisture function. Thus, if moisture and temperature are optimum for decomposition on a given day, a k of e.g. 0.01 results in a 1% reduction of stem mass on this day. TF and MF range between 0 and 1 and depend on residue placement (standing, flat or buried).

For standing residue the moisture function is (Steiner et al., 1994):

where MFs, t-1 is the moisture function for standing residue on the previous day. It is based on the precipitation P (mm), with 4 mm of precipitation considered to saturate the standing residue. MFs decreases by 60 % each day following a wetting event. After more than 4 dry days in a row MFs = 0 (Figure 2).

The decomposition of flat residue is influenced by both precipitation and soil moisture at the soil surface. The maximum of either the above estimate of MFs or a function MFf that considers the flat residue to be in equilibrium with the soil surface is used. MFf is calculated from the soil water content at the soil surface at noon, surf, and the optimum water content for decomposition. Water content at field capacity of the upper soil layer, fc, 1, is considered to be the optimum water content:

If flat leaves are present in the crop pool (see Crop submodel), they will be decomposed in the same manner as flat residue.

For buried residue and roots there is a moisture function for each soil layer:

where i is the water content of layer i and fc, i is the water content at field capacity of layer i.

The temperature function is calculated similarly to a function describing the influence of temperature on photosynthesis (Taylor and Sexton, 1972) and used by Stroo et al. (1989) and Steiner et al. (1999) for residue decomposition:

where T is temperature (°C), To is the optimum temperature for decomposition (32 °C) and Tb is a base temperature (0 °C) below which no decomposition occurs (see Figure 3). For standing and flat residue TF is calculated as the average of two TF values: one calculated using daily maximum air temperature and a second using daily minimum air temperature. For below ground residue daily average soil temperature by layer is used.

Decline in Stem Population and Standing Biomass

Standing residue losses not only result from microbial activity, but also from physical forces (Steiner et al, 1994), reducing both the stem population and standing biomass. Standing residue losses due to physical forces such as wind, snow, gravity, or wheel traffic result in transfer to the flat pool. Tillage may result in redistribution to both the flat and the buried pools. In addition, tillage may result in accelerated stem fall (see Management submodel). A daily estimate of the standing stem population is required in order to evaluate stem area index and its influence on aerodynamic resistance and, ultimately, wind erosion.

Stems are allowed to fall over only after a threshold of cumulative decomposition days since harvest has been reached. For example, a threshold of 20 decomposition days means that standing stalks begin to fall 20 days after harvest if moisture and temperature conditions are optimum during these 20 days. If conditions are not optimum, the number of days that stalks remain standing increases. After reaching the threshold, the change in stem number is calculated similarly to the loss in biomass (equation 1.2),

where Nt is the number of stems standing on day t (# m-2), Nt-1 is the number of stems standing on the previous day (# m-2), and kstem is a crop specific rate constant (# #-1 day-1) (Steiner et al, 1994). The standing residue provides a vertical area that has a direct influence on wind speed. Stem area index (SAI, m2 m-2) decreases proportionally with stem number. For the three age pools, SAI is estimated by

where h is stem height (m) and D is stem diameter (m) (Figure 4).

Soil Cover

Both standing and flat crop residue provide cover to the soil surface. Soil cover from standing residue is typically small. For the three age pools, it is estimated as:

where Cs is standing residue cover (m2 m-2) (Figure 5). Soil cover from flat residue mass is predicted by an equation developed by Gregory (1982) and used by Steiner et al. (2000):

where Cf is flat residue cover (m2 m-2), b is a crop specific coefficient (m2 kg-1), and Mf is flat residue mass (kg m-2) (Figure 6). Residue from crops such as cotton and sunflower (b = 2.1 m2 kg-1) does not provide much cover per unit mass, whereas wheat residue provides much more cover (b = 6.5 m2 kg-1). Total flat cover from the three age pools is calculated, taking into account overlap among the three age pools. Total (standing plus flat) residue cover is computed by adding total standing residue cover and total flat residue cover.

Changes Due to Tillage Operations

On a day of tillage, the distribution of residue will change between standing, flat and buried components depending on the tillage implement being used. The Management submodel updates current biomass for each position (standing, flat, buried, and root) in each of the three age pools. Soil surface cover is then updated from the amount of biomass remaining in the flat and standing pools.

Gregory, J.M. 1982. Soil cover prediction with various amounts and types of residue. Trans. ASAE. 25: 1333-1337.

Steiner, J.L., H.H. Schomberg, C.L. Douglas, Jr., and A.L. Black. 1994. Standing stem persistence in no-tillage small-grain fields. Agron. J. 86:76-81.

Steiner, J.L., H.H. Schomberg, P.W. Unger, and J. Cresap. 1999. Crop residue decomposition in no-tillage small-grain fields. Soil Sci. Soc. Am. J. 63: 1817-1824.

Steiner, J.L., H.H. Schomberg, P.W. Unger, and J. Cresap. 2000. Biomass and residue cover relationships of fresh and decomposing small grain residue. Soil Sci. Soc. Am. J. 64: 2109-
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Stroo, H.F., K.L. Bristow, L.F. Elliott, R.I. Papendick, and G.S. Campbell. 1989. Predicting rates of wheat residue decomposition. Soil Sci. Soc. Am. J. 53:91-99.

Taylor, S.E. and O.J. Sexton 1972. Some implications of leaf tearing in musacea. Ecology 53: 143-149.