Effect of crop type and cultivar surface area on rates of decomposition in soils

Sustaining the productive capacity of soils has raised interest in the maintenance of soil organic matter through management practices and use of crop residues. While the impact of management practices has been studied, little is done to understand how the charateristics of the residue itself impact the decomposition at the soil surface. This study relates the chemical composition and the surface area of the aboveground residue to the decomposition rates for three cultivars each of three crops: cotton, peanut and sorghum. The rates were determined by mass loss. Change in the residue specific surface area to mass loss was also measured. Findings show that after 14 days, the aboveground residue for the three crops were from the most rapid loss to the slowest: cotton (43%) > peanut (32%) > sorghum (24%). Changes in the specific surface area-to-mass ratio were from the slowest to the most rapid loss: cotton (1.60×10 -4 ) > peanut (1.50×10 -4 > sorghum (1.20×10 -4 ). Since varietal differences within crops have led to variation in decomposition rates, cultivars with slower decaying residues might be recommended for C sequestration and for erodible lands in semi-arid zones of the Sahel. Likewise, crop residues with faster decomposition rates can be recommended for soil fertility improvement.


INTRODUCTION
Maintaining crop residue on the soil surface is an effective and cost-effective practical method for controlling wind and water erosion.It provides a large potential to sequester C in the soil, which may be preferable to storage in vegetation due to their longer residence times and less risk of a rapid release (Lal et al., 1999).It also offers an outlook on targeted strategies for cropping and farming systems to cope and adapt to climate change and variability, as well as soil fertility challenges within the socio-ecological context (Callo-Concha et al., 2013).
In many areas of the world, insufficient amounts of residue are produced to provide adequate erosion protection.While in some areas, the accumulation of crop residues is frequently viewed as a nuisance to crop establishment and growth, and a disposal problem, in other areas, there are not enough surface residues due to low productivity, burning for management purposes or utilization as animal feed or even fuel (Diack et al., 2000).
In West Africa, cotton, peanut and sorghum crops cover 50 to 60% of the rainfed areas (Laube, 2007), which explains why they have been chosen for this study.Through its importance as a cash crop, cotton (Gossypium hirsutum), has received wide attention from the African governments, especially in the Benin and Burkina Faso (Slingerland, 2000).The Centre de Cooperation International en Recherche Agronomique pour le Developpement (CIRAD) reported good adoption of improved varieties, mineral fertilization, phytosanitary measures and animal traction in the framework of (a) close research-extension-farmer relationship, (b) provision of input credits, and (c) guarantee of market outlets (The World Bank, 2002;Gray, 2005;Gaiser et al., 2010).
Peanut (Arachis hypogea) is the main legume cultivated in the Sudan Savanna and, together with other leguminous species, makes large contribution to fulfilling the protein demand of the local population and the provision of high quality fodder for livestock (Slingerland, 2000).Peanut is a preferred legume due to its ability to produce well under soil-moisture-deficient conditions, as well as being a source of external income since it is well sold in the market and even exported (Ntare et al., 2007).
Sorghum (Sorghum bicolor, Sorghum vulgare), commonly called "guinea corn" or "red millet", is widely cultivated.It originated in eastern Africa where its major variability can be found.Accordingly, sorghum has developed various morphological and physiological adaptations, such as drought resistance.It performs well at rainfall levels (400 to 600 mm/year) too low for maize.The response of sorghum to management is diverse and depends on the variety.Local varieties are poorly responsive, but improved ones respond well to fertilization.Normally it is cultivated in combination with other crops (Schipprack and Abdulai, 1992).
Crop residues are an important source of organic matter that can be returned to soil for nutrient recycling, and improve soil physical, chemical and biological properties (Kumar and Goh, 2000).They contain all mineral nutrients, the content of which varies among crop species depending on the fertility of the soil (Brennan et al., 2004).These residues should be returned to the soil and should be spread uniformly over an entire field to prevent nutrient and organic C in the soil (Lal, 2005).Given that a decrease in soil fertility is a major constraint to productivity, investing in practices leading to soil fertily enhancement is likely to general large returns.In recent years, increased concerns for healthy food production and environmental quality, and increased emphasis on sustaining the productive capacity of soils, have raised interest in the maintenance and improvement of soil organic matter through appropriate land use and Diack and Stott 5125 management practices (Loveland and Webb, 2003;Puget and Lal, 2005;Whitbread et al., 2003).
It is however, difficult to predict how much of the nutrients in the residue will become available to crop during a given time because of the complex processes governing residue decomposition and nutrient release (Iyamuremye et al., 2000).In addition, the nature of crop residue and their management can significantly affect the amount of nutrients available for subsequent crop as well as the content and quality of soil organic matter (Kumar and Goh, 2000;Yadvinder-Singh et al., 2005).Effective management of crop residues in the field should conserve soil and its resources with minimal adverse effects on the environment (Conteh et al., 1998;Puget and Lal, 2005).For most soils, the higher the level of crop residue (stems, stalks, and leaves from the previous harvest) left on the surface of a field, the greater the benefits.
However, to optimize this effect, fundamental information is needed on residue decomposition and how the characteristics of the residue itself impact the decomposition rate.Wickings et al. (2012), found out that the chemical complexity of decomposing plant litter is a central feature shaping the terrestrial carbon (C) cycle, but explanations of the origin of this complexity remain contentious.How does litter chemistry change during decomposition and what roles do decomposers play in these changes?The rate of residue decomposition will determine the amount of soil surface covered during critical erosion periods throughout the year, as well as the amount of residues in top portion of the soil profile.
Plant residues consist of two parts: the aboveground portion, mainly composed of stems and leaves, and the roots.The aboveground biomass may be standing flat on the soil surface, or become buried through tillage and other management practices.The physical nature and the initial chemical composition of the plant residues largely determine the ability of microorganisms to assimilate them.In the traditional agronomic literature, the C/N ratio has been assumed to be a controlling factor, while in the traditional forestry literature, the ligninto-N has been considered most important (Abril and Bucher, 2001).However, the C/N ratio is apparently not the determining factor, nor is the lignin-to-N ratio solely responsible (Dempsey et al., 2013).Decomposition rate for plant residue varies between plant species and between cultivars within species (Stott, 1993).Most knowledge about crop residue decomposition is based on above-ground residue, mostly winter wheat.Increased soil organic matter (SOM) in semi-arid environments, through optimal soil management practices, could be beneficial to food productivity and erosion control in poor and degraded areas, in addition to the removal of atmospheric CO 2 (Ringius, 1999).This practice may be new to most smallholder farmers.In semi-arid areas, crop residues serve as forage for livestock during the dry season.The land use right of the farmer is limited to the growing season.Later, the fields are opened for common grazing.Apart from this, crop residues serve also as construction material or fuel.To change this situation, farmers have to be convinced of the advantage of leaving residues in the field to cover the soil surface and alternatives have to be shown.
The specific-surface-area-to-mass ratio (k) represents a fraction of an area (ha) of soil covered by one kg of residue and is specific for a crop type.The k value is a conversion constant (ha kg -1 ) used in an equation for converting residue mass to cover (Gregory, 1982): where C = fraction of the surface cover remaining and m = mass (kg ha -1 ) of residue present on the surface.The Gregory equation is currently used in all the USDA erosion MODELS: WEPP (Water Erosion Prediction Project), WEPS (Wind Erosion Prediction System), RUSLE (Revised Universal Soil Loss Equation), and RWEQ (Revised Wind Erosion Equation).
The residue mass-surface cover relationship is closely related to the levels of residues, and considerable decomposition of mass may occur before a large decrease in cover is measured (Steiner et al., 1993).For residues having high proportion of leaf material following harvest, there may be tremendous loss in mass with little loss in cover, because leaf material decomposes rapidly and is light compared to stem material.Stem will loose mass, not surface area.
Therefore, understanding the mechanisms of residue decomposition is necessary for developing a viable crop residue management system for a better land management leading to a sustainable agricultural productivity and ultimately food security.The objectives of this study were to: (i) determine decomposition rates for cotton, peanut and sorghum above-ground residues by mass loss; (ii) determine how initial physical and chemical properties of the residues impact the decomposition rates; (iii) determine if differences in decomposition exist between plant cultivars within a species; and (iv) determine changes in the mass-tospecific surface area during decomposition.

Soil
A Russell silt-loam (fine-silty, mixed, mesic Typic Hapludalf) soil was used in this study.It was obtained from the Ap horizon at the Purdue Agronomy Research Center in West Lafayette, IN.The soil was air-dried (to minimize microbial action before use), crushed to pass a 2-mm mesh screen, and then stored until use.The soil had a pHwater of 5.3, a total C content of 7.8 g kg -1 , and a total N content of 1.2 g kg -1 .

Plant materials
Plant from three field-grown crops: cotton (G.hirsutum), peanut (A.hypogaea) and sorghum (So.bicolor) were collected at maturity.Each crop was represented by three genetically different cultivars.These cultivars are the following: cotton (DLP-5660, DP-5215 and HS-46), peanut (Florunner, NC-7 and NC-11) and sorghum .For each cultivar, the plant material was collected from the aboveground residue (leaves and stems).These components were used to determine the residue decomposition rate.Plant residue samples were collected by USDA-SCS personnel from fields in several states, within one or two days of harvest in order to be in unweathered condition and maximize their use.Five plant samples, representative of the whole field, were selected: one from the center and four from 4 corners, avoiding the end rows.When removing the whole plant from the ground, care was taken so that the roots within the top 10 to 20 cm of the soil did not break apart.The residues were shipped overnight to the National Soil Erosion Research Laboratory (NSERL) in West Lafayette, IN.The leaves and stems (above-ground biomass) were separated from the roots.The residues were gently washed with water to remove any remaining soil and air-dried before chemical analysis.These components were used to determine the residue decomposition rates and the surface area.

Composition analysis
Each plant residue component was chemically analyzed for total C content, total N content, simple sugar content and structural and non-structural contents.Total C and N were measured by dry combustion (Model CHN-600; Leco Corp., St Joseph, MI).Hemicellulose, cellulose and lignin contents were determined by sequential fiber analysis (Goering et al., 1970).This fiber analysis system was designated to provide estimates of forage fiber composition.Sucrose and fructose were measured colorimetrically from a 1:1 weight-volume ratio of finely ground residue and 50% ethanol solution.For sucrose, 100 µl of 30% KOH was added to destroy the sugars, whereas for fructose, 3 ml of concentrated HCl was added plus 1 ml of 0.05% resorcinol reagent.

Plant residue mass loss experiment
The mass loss experiment consisted of a split-split plot design of 3 crop types as main plots and 3 cultivars as subplots; leaves and stems as the last split.Treatments were in triplicate.Each treatment consisted of leaves and stems in the same proportion as was present in the aboveground biomass after harvest.
Residues were cut into 4 to 5-cm long and the pieces were spread evenly on the soil surface in a 10 by 7.5 cm 2 polystyrene dish.Optimum moisture conditions were assumed to be the water content at -1/3 bar water potential as equalled to 60% water holding capacity, plus 300% of the residue mass (Myrold et al., 1981).After the appropriate amount of water was added, the incubation dish was loosely wrapped with a food service film (PYA/Monarch, Inc., Greenville, SC), to allow some aeration.The samples were incubated at 22 ± 1°C.
Samples were withdrawn on days 3, 7, 14, 28, 56 and 84 of the incubation for mass measurement.At each destructive sampling, the incubation mixture was oven-dried at 40°C, for 48 h.When dry, the residues were carefully separated from the soil, gently washed to remove the soil particles and put back into the oven at 40°C for 48 h.The residues were weighed then placed into crucibles for ashing at 800°C for 2 h.

Measurement of specific surface area-to-mass ratio
Specific surface areas for the leaves and stems were measured using a digitizer (Summagraphics) and AutoCad.As decomposition proceeded, the ration between the specific surface area and the mass remaining was calculated at each sampling time.
The equation used to convert residue mass to cover is from Gregory (1982): (-km)  (1.0) where C is the fraction of the surface cover remaining and m is the mass (kg ha-1) of residue present on the surface.
The constant k can be derived from the following equation:

Statistical analysis
Statistical analysis of the data was done to determine differences among treatments, using the PC-SAS, Version 9.01 (SAS Inc., Cary, NC).Comparisons between treatment means were made at the P=0.05 level using the Student-Newman-Keuls"s multiple range test procedure.

Initial chemical composition
The mean concentrations of total C and N, simple sugars, hemicellulose and lignin (Table 2) were significantly different between the aboveground biomass residues for cotton cultivars DLP-5690, DP-5215 and HS-46.Total C, total N, hemicellulose and lignin contents were 103, 163, 190, and 139% greater, respectively for DLP-5690 than for DP-5215, whereas, simple sugars content was 78% lower.For HS-46 aboveground residues, the mean concentrations of total C and N, simple sugars, hemicellulose and lignin were 105, 160, 147, 197 and 128% higher, respectively than for DP-5215.
As for sorghum, the mean concentrations of total C and N, simple sugars, hemicellulose and lignin were significantly different between the aboveground biomass residues for Triumph-266, GW7-44BR and NKing-300.Total C and N and hemicellulose contents were 103, 150 and 157% greater, respectively for GW7-44BR than for Triumph-266, simple sugars content was 26% lower.For NK-300 aboveground residues, the mean concentrations of total C and N, hemicellulose and lignin were 2, 38 and 84% lower respectively than for GW7-44BR, whereas, simple sugars content was 150% greater.Table 3 indicated significant differences in initial chemical composition between cultivars within crops.

Initial specific surface area
For cotton, the specific surface area (Table 3) of the aboveground residues before incubation is not significantly different between cultivars (Figure 6).The specific surface area of DLP-5690, DP-5215 and HS-46 leaves was 101, 73 and 85 greater than the stems, respectively.No peanut cultivar was significantly different from one another for the aboveground specific surface area (Figure 7).However, the specific surface area of the leaves was significantly greater than the stems by 95% for Florunner, 235% for NC-7, and 113% for NC-11.As for sorghum, the initial specific surface area of the leaves and stems (Table 3) showed significant differences between cultivars except for GW-744BR.Triumph-266 leaf specific surface area was greater by 45% than that of the stems.GW-744BR leaf specific surface area was not significantly different from that of the stems (Figure 8).In the other hand, NKing-300 leaf specific surface area was 87% higher than that of the stems.
For Triumph-266, the specific surface area was 18% greater than that of GW-744BR, but 9% lower than that of NKing-300.As for GW-744BR, the leaf specific surface area was 23% lower than that of NKing-300.

Initial aboveground residue mass
For all crops and each cultivar, aboveground residue mass was a combination of stem and leaves with different proportions (Table 1).Within cotton, cultivar HS-46 aboveground residue mass was higher than those of DLP-5690 and DP-5215 cultivars.For peanut, there was no significant difference in aboveground biomass between cultivars.On the other hand, GW-744BR sorghum cultivar, presented a greater aboveground residue mass than those of Triumph-266 and NKing-300.

Change in mass loss
In determining mass loss, the aboveground residues composed of leaves and stems (Table 1), were monitored in terms of changes in mass loss.For cotton cultivars, the rate of mass loss of the aboveground residues was significantly different between cutivars (Figure 2).HS-46 had a faster breakdown rate, 38%, followed by that of DP-5215, 30% and DLP-5690, 26%.Peanut aboveground residue mass loss did not present any significant difference from one cultivar to another in the percent mass remaining during the first 14 days (Figure 3).As for sorghum (Figure 4), cultivars showed 32% of mass loss for Triumph-266, 24% for GW-744BR and 20% for NKing-300 in the early decomposition phase.Triumph-266 cultivar presented a significant difference in aboveground residues mass loss compared to the other two cultivars (Figure 4).There was no difference in decay rates between the aboveground residues for the three cultivars (Figure 4).Significant differences in mass remaining were observed between the mean mass loss of the cultivars of cotton, peanut and sorghum aboveground biomass in the early decomposition phase.Overall, cotton mean residue mass loss was greater, 45%, than those of peanut, 40%, and sorghum, 34% (Figure 1).

DISCUSSION
The decomposition rates for all cotton (Figures 1 and 2), peanut (Figures 1 and 3), and sorghum (Figures 1 and 4) cultivars followed the pattern for Michaelis-Menten firstorder kinetics.The rapid increase in mass loss during the first 14 days was probably due to the high total N content, the high level of readily available C in the form of extractible sugars or a combination of the two (Table 3).Kinetically, the mass loss from the residues studied exhibited a linear dependence on the chemical composition of the residue.The rapid disappearance of these soluble compounds was probably related to a quick    build up of the microbial activity, which would increase the mass loss.Also, the readily available C and N components in the crop residues might provide the initial energy and nutrients necessary to activate the microorganisms that are responsible for the degradation of the less readily available components of the residue (Sall et al., 2007).The levelling off phase of the mass loss, from all three crops, between days 15 and 28, would be the period during which hemicellulose was the main fraction available to the microorganisms (Figures 1, 2, 3  and 4).As decomposition process proceeds, the mass loss slows down, following an exponential trend, probably due to change in chemical composition of the remaining residue available to the microorganisms (Rinkes et al., 2013).In this phase of the decomposition, the hemicellulose fraction probably disappears initially at a rapid rate, but the subsequent degradation appears to be slower.Residue recalcitrance controls decomposition and soil organic matter turnover (Machinet et al., 2009).In addition, the presence of lignin and cross-linking phenolic acids is well known to regulate enzyme access to cellulose and hemicelluloses in forage digestibility and bioreffinery studies (Lam et al., 2003;Berlin et al., 2006) and appears to affect decomposition in soils (Machinet et al., 2011b;Talbot et al., 2012).Such degradation of hemicellulose is more marked when the environment is aerobic, and when there is availability of inorganic nutrients, especially nitrogen.At this stage of the decomposition process, there is probably not enough N or readily available C to keep the microbial activity at high level.As a result, there is a decrease in decomposition rate, resulting in a slower rate of mass loss (Elliott al., 1986;Stott, 1993;Iqbal et al., 2014).All residues types show the same trend and similar slopes in this portion of the curve, suggesting that the second phase of the decomposition is probably not a good element of comparison of mass loss.After 28 days of decomposition process, the remaining residues entered the third phase of the decomposition process.At this point, the slowly available residue components dominated the residue substrate.Lignin, known to be resistant to degradation, was probably the major remaining component.The rate and extent of lignin decomposition are affected by temperature, availability of nitrogen, and by constituents of the residues undergoing decay.At this stage of degradation, all the readily available nutrients are expected to vanish.Lignin is probably being decomposed by relatively slowly growing microorganisms.Consequently, microbial activity is very low.As a result, mass loss follows a quasi steady state for the rest of the decomposition.Lignin continues to disappear however.Cotton cultivar DPL-5690 and DP-5215 aboveground biomass (Figure 2) showed a great cumulative mass loss due to total N, lower hemicellulose and lignin cooncentation in the residue.In addition, lower lignin content plus high specific area-to-mass ratios for the aboveground residue provide with microorganisms better access to available C sources (Jensen, 1994).As for the HS-46 cultivar aboveground residues, the specific surface area-to-mass was probably too low in aboveground biomass to provide with microorganisms good access to available C sources.For all peanut cultivars (Figure 3), the aboveground residues showed quite high cumulative mass loss due higher simple sugar contents available to the microorganisms, combined with lower lignin concentration of the aboveground biomass.There was no difference in rate of breakdown of the aboveground residues between the three cultivars.The insignificant difference in sugar concentrations between Florunner, NC-7 and NC-11 aboveground residues (Table 2) certainly excludes any difference in their cumulative mass loss (Figure 3).Peanut is a legume, and the highest N level is concentrated not in the aboveground biomass, probably in the root system where the pods are produced.
The sorghum cultivar, GW7-44BR (Figure 4), showing a significant difference in mass loss between the aboveground residues had the highest total N, and the lowest simple sugars and lignin concentrations in the aboveground residues.For Triumph-266 and NKing-300 (Figure 4), high available C in the form of simple sugar concentration, associated with low hemicellulose content probably contributed to their higher mass loss level for the aboveground residues.These results were consistent with who observed that high levels of sugars in sorghum furnished the energy for multiplication of soil microorganisms, which compete with plants for the available soil nitrogen.The data (Tables 2 and 3) support the differences in cumulative mass loss among residues.These results agreed with Collins et al. (1993) data in their study of decomposition of winter wheat residues.great increasing trend of breakdown.
Specific surface area-to-mass relationships, represented by a k value, is a specific surface area-to-mass ratio with dimension of ha kg -1 of residue.In Gregory"s (1982) Equation 2, k is specific for a given crop and considered to be constant over time.Between cotton, peanut and sorghum crops, k was noticed to be significantly different (Figure 5).The initial k value for cotton was greater, 42×10 -5 , than peanut and sorghum, 23×10 -5 and 18×10 -5 , respectively.In the first 14 days, change in specific surface area-to-mass ratio was relatively rapid for cotton and peanut residues but change in sorghum was quite slow.Stott et al. (1994) found a k value of 23×10 -5 for corn from field data.This was consistent with the range of values from this study as the three crop species used, sorghum is the crop that is physiologically and morphologically closest to corn and both are monocotyledons.Compared to corn, sorghum has a lower osmotic concentration of the leaf juices, but the stalks, crown and root juices are higher in sorghum (Leonard et al., 1963).In addition to its juicy stem, sorghum leaf area is smaller than that of corn.Therefore, sorghum residue decomposition may be somewhat faster    than corn.Consequently, a k value for sorghum should be lower, but close to that of corn residue.k was found to be a value specific to each crop species.It changes within a certain range over time, during the decomposition process because it is a ratio of specific surface area over mass of the decomposing residue (Equation 2).In this study, significant differences were observed between cultivars for cotton in the first 10-14 days, but to a lesser extent for peanut and sorghum cultivars.However, such significant difference in mean k values between cotton, peanut and sorghum species was consistent with its specificity to each crop (Stott, 1993).

Conclusion
The initial chemical and physical characteristics of the aboveground residues impacted the rates of decomposition.The decomposition rates determined by mass loss showed differences between cultivars, for cotton, peanut and sorghum.Due to their leguminous nature, the three peanut cultivars were decomposed rapidly, and were different in decay rates between them.The degradability of peanut aboveground residue was highest followed by cotton, while the sorghum aboveground decomposition fate was the slowest.The different decomposition rates for each crop did follow the same order in degradability for the aboveground residues.There was significant difference between the decomposition rates of the cotton and peanut roots.Changes in specific surface area-to-mass measurements showed significant différences between cultivars within cotton only, but there were differences between species as if k value was a constant specific for each crop.Determining residue decomposition, as used in a management program, can help solve soil degradation problems in the semi arid zones.
the same letter, within crops, are not significantly different by the Waller-Duncan"s multiple range test at P = 0.05.
the same letter, within crops, are not significantly different by the Waller-Duncan"s multiple range test at P = 0.05.

Figure 1 .
Figure 1.Changes in aboveground residue mass loss over time for cotton, peanut and sorghum crop species.Bars represent standard deveiations at given time.

Figure 2 .
Figure 2. Changes in aboveground residue mass loss over time for cotton crop species.Bars represent standard deveiations at given time.

Figure 3 .
Figure 3. Changes in aboveground residue mass loss over time for peanut crop species.Bars represent standard deveiations at given time.

Figure 4 .
Figure 4. Changes in aboveground residue mass loss over time for sorghum crop species.Bars represent standard deveiations at given time.

Figure 5 .
Figure 5. Specific surface area-to-mass ratio for cotton, peanut and sorghum aboveground residues over time.Bars represent standard deviations at given time.

Figure 6 .
Figure 6.Specific surface area-to-mass ratio for cotton aboveground residues over time.Bars represent standard deviations at given time.

Figure 7 .
Figure 7. Specific surface area-to-mass ratio for peanut aboveground residues over time.Bars represent standard deviations at given time.

Figure 8 .
Figure 8. Specific surface area-to-mass ratio for sorghum aboveground residues over time.Bars represent standard deviations at given time.

Table 1 .
Loading rates of crop residues added to soil for decomposition study.

Table 2 .
Initial chemical composition of the aboveground biomass residues.

Table 3 .
Relative initial mass and specific surface area of the residue components.