Humberto Blanco-Canqui

Mechanisms of Carbon Sequestration in Soil Aggregates

Soil and crop management practices have a profound impact on carbon (C) sequestration, but the mechanisms of interaction between soil structure and soil organic C (SOC) dynamics are not well understood. Understanding how an aggregate stores and protects SOC is essential to developing proper management practices to enhance SOC sequestration. The objectives of this article are to: (1) describe the importance of plants and soil functions on SOC sequestration, (2) review the mechanisms of SOC sequestration within aggregates under different vegetation and soil management practices, (3) explain methods of assessing distribution of SOC within aggregates, and (4) identify knowledge gaps with regards to SOC and soil structural dynamics. The quality and quantity of plant residues define the amount of organic matter and thus the SOC pool in aggregates. The nature of plant debris (C:N ratio, lignin content, and phenolic compound content) affects the rate of SOC sequestration. Mechanisms of interaction of aggregate dynamics with SOC are complex and embrace a range of spatial and temporal processes within macro- ( > 250 μ m e.c.d.) and microaggregates ( < 250 μ m e.c.d.). A relevant mechanism for SOC sequestration within aggregates is the confinement of plant debris in the core of the microaggregates. The C-rich young plant residues form and stabilize macroaggregates, whereas the old organic C is occluded in the microaggregates. Interactions of clay minerals with C rich humic compounds in correlation with clay mineralogy determine the protection and storage of SOC. Principal techniques used to assess the C distribution in aggregates include the determination of total organic C in different aggregate size fractions, isotopic methods to assess the turnover and storage of organic C in aggregates, and computed tomography and X-ray scattering to determine the internal porosity and inter-aggregate attributes. The literature is replete with studies on soil and crop management influences on total organic C and soil aggregation. However, research reports on the interactions of SOC within aggregates for C sequestration are scanty. Questions still remain on how SOC interacts physically and chemically with aggregates, and research is needed to understand the mechanisms responsible for the dynamics of aggregate formation and stability in relation to C sequestration.

Rapid Changes in Soil Carbon and Structural Properties Due to Stover Removal from No-Till Corn Plots

Harvesting corn (Zea mays L.) stover for producing ethanol may be beneficial to palliate the dependence on fossil fuels and reduce CO2 emissions to the atmosphere, but stover harvesting may deplete soil organic carbon (SOC) and degrade soil structure. We investigated the impacts of variable rates of stover removal from no-till (NT) continuous corn systems on SOC and soil structural properties after 1 year of stover removal in three soils in Ohio: Rayne silt loam (fine-loamy, mixed, active, mesic Typic Hapludults) at Coshocton, Hoytville clay loam (fine, illitic, mesic Mollic Epiaqualfs) at Hoytville, and Celina silt loam (fine, mixed, active, mesic Aquic Hapludalfs) at South Charleston. This study also assessed relationships between SOC and soil structural properties as affected by stover management. Six stover treatments that consisted of removing 100, 75, 50, 25, and 0, and adding 100% of corn stover corresponding to 0 (T0), 1.25 (T1.25), 2.50 (T2.5), 3.75 (T3.75), 5.00 (T5), and 10.00 (T10) Mg ha−1 of stover, respectively, were studied for their total SOC concentration, bulk density (&rgr;b), aggregate stability, and tensile strength (TS) of aggregates. Effects of stover removal on soil properties were rapid and significant in the 0- to 5-cm depth, although the magnitude of changes differed among soils after only 1 year of stover removal. The SOC concentration declined with increase in removal rates in silt loams but not in clay loam soils. It decreased by 39% at Coshocton and 30% at Charleston within 1 year of complete stover removal. At the same sites, macroaggregates contained 10% to 45% more SOC than microaggregates. Stover removal reduced >4.75-mm macroaggregates and increased microaggregates (P < 0.01). Mean weight diameter (MWD) and TS of aggregates in soils without stover (T0) were 1.7 and 3.3 times lower than those in soils with normal stover treatments (T5) across sites. The SOC concentration was negatively correlated with &rgr;b and positively with MWD and LogTS. Stover removal at rates as low as 1.25 Mg ha−1 reduced SOC and degraded soil structure even within 1 year, but further monitoring is needed to establish threshold levels of stover removal in relation to changes in soil quality.

Soil and crop response to stover removal from rainfed and irrigated corn

Excessive corn (Zea mays L.) stover removal for biofuel and other uses may adversely impact soil and crop production. We assessed the effects of stover removal at 0, 25, 50, 75, and 100% from continuous corn on water erosion, corn yield, and related soil properties during a 3‐year study under irrigated and no‐tillage management practice on a Ulysses silt loam at Colby, irrigated and strip till management practice on a Hugoton loam at Hugoton, and rainfed and no‐tillage management practice on a Woodson silt loam at Ottawa in Kansas, USA. The slope of each soil was <1%. One year after removal, complete (100%) stover removal resulted in increased losses of sediment by 0.36–0.47 Mg ha−1 at the irrigated sites, but, at the rainfed site, removal at rates as low as 50% resulted in increased sediment loss by 0.30 Mg ha−1 and sediment‐associated carbon (C) by 0.29 kg ha−1. Complete stover removal reduced wet aggregate stability of the soil at the irrigated sites in the first year after removal, but, at the rainfed site, wet aggregate stability was reduced in all years. Stover removal at rates ≥ 50% resulted in reduced soil water content, increased soil temperature in summer by 3.5–6.8 °C, and reduced temperature in winter by about 0.5 °C. Soil C pool tended to decrease and crop yields tended to increase with an increase in stover removal, but 3 years after removal, differences were not significant. Overall, stover removal at rates ≥50% may enhance grain yield but may increase risks of water erosion and negatively affect soil water and temperature regimes in this region.

Wheat and sorghum residue removal for expanded uses increases sediment and nutrient loss in runoff.

Crop residue removal for expanded uses such as feedstocks for cellulosic ethanol production may increase loss of sediment and nutrients in runoff. We assessed on-farm impacts of variable rates of residue removal from no-till winter wheat (Triticum aestivum L.) and plow till grain sorghum [Sorghum bicolor (L.) Moench] on sediment, soil organic carbon (SOC) and nutrient losses in runoff in western Kansas. Five treatments with three replications consisting of removing residues at 0, 25, 50, 75, and 100% after harvest under two tillage levels for wheat (no-till and freshly tilled) and grain sorghum (spring tilled and freshly tilled) were established on 1x2 m plots. Simulated rainfall was applied at 115+/-3 mm h(-1) for 30 min. Compared with plots without residue removal, complete removal increased runoff by 61% in freshly tilled wheat plots, 225% in spring-tilled sorghum plots, and 94% in freshly tilled sorghum plots. Residue removal at rates as low as 50% increased loss of sediment. Complete removal doubled the sediment loss to 14 Mg ha(-1) in tilled wheat, whereas it increased sediment loss from 0.9 to 7.2 Mg ha(-1) in no-till wheat. No-till with 100% residue removal lost as much sediment as freshly tilled wheat plots with 0 or 25% removal. Residue removal at 75 and 100% increased losses of total N, total P, and SOC associated with sediment. Overall, excessive residue removal led to large losses of sediment, sediment-bound SOC, and nutrients in runoff. Furthermore, erosion protection provided by no-till management is lost when residue removal exceeds 25%.

Soil organic carbon: The value to soil properties

Maintaining or improving soil properties is becoming increasingly important to sustain modern agriculture under increasing demands to preserve biodiversity and environmental quality. Enhancing the inherent capacity of a soil to buffer changes against anthropogenic stresses and extreme climatic events such as droughts, intense rainstorms, heat waves, and floods is also a priority. Managing soil organic carbon (SOC) through optimized management practices is one strategy to enhance soil ecosystem services. Increasing organic C storage in the soil not only sequesters atmospheric C but often enhances soil physical, chemical, and biological processes and properties. Soil organic C has been widely discussed in terms of C sequestration, but its benefits on soil processes and properties have received less attention in recent years. Thus, this article discusses (1) the value of SOC to soil properties and (2) potential for increasing SOC through management. SOIL CARBON DYNAMICS The balance between C inputs and outputs is important to SOC change. Inputs include aboveground and belowground crop residues, animal manure, compost, and others, whereas outputs are losses through water and wind erosion, gas fluxes associated with microbial and plant respiration, and deep leaching. How fast residues decompose following the well-known exponential decay function depends…

Soil aggregate properties and organic carbon for switchgrass and traditional agricultural systems in the Southeastern United States.

Switchgrass (Panicum virgatum L.), a potential biofuel crop, can sequester soil organic carbon (SOC) and improve soil quality. However, its influence on soil aggregate mechanical properties controlling the macro-scale behavior of the whole soil needs to be assessed to understand processes that affect soil quality. This study assessed the impact of long-term (>10 years) switchgrass, row crop, cool season grass pasture, and forest management on properties of soil aggregates for five ecosystems in the southeastern United States, including Blacksburg and Orange (VA), Knoxville (TN), Morgantown (WV), and Raleigh (NC). Relationships among aggregate properties were also determined. Tensile strength (TS), bulk density (&rgr;agg), soil moisture retention (SMR), and SOC concentration of 1- to 8-mm aggregates were determined at the 0-to 10-cm and 10- to 20-cm soil depths. Management significantly affected the aggregate properties (P < 0.05), but the magnitude of the effects was site-dependent. The TS for switchgrass was the lowest (∼271 kPa) at all but the Blacksburg site for the 0- to 10-cm depth. The &rgr;agg for switchgrass was 10% lower at Blacksburg and 20% lower at Orange than that for row crop at the 0- to 10-cm depth. The SOC concentration for switchgrass was 2.5 times higher than that for row crop at Orange but not at Blacksburg. The TS increased with increasing &rgr;agg at Morgantown and Raleigh, but it decreased with increasing aggregate size at all sites. Aggregate size, &rgr;agg, and SOC were significant predictors of TS. Long-term switchgrass systems in the southeastern United States improve the aggregate strength properties, unlike row crop and cool season grass pastures, but their impact on SOC concentration is variable.

Soil Aggregate Stability and Aggregate-Associated Carbon Under Different Tillage Systems in the North China Plain

Abstract The influences of tillage systems on soil carbon (C) stocks have been studied extensively, but the distribution of soil C within aggregate fractions is not well understood. The objective of this study was to determine the influences of various tillage systems on soil aggregation and aggregate-associated C under wheat ( Triticum aestivum L.) and corn ( Zea mays L.) double cropping systems in the North China Plain. The experiment was established in 2001, including four treatments: moldboard plow (MP) with residue (MP+R) and without residue (MP-R), rotary tillage with residue (RT), and no-till with residue (NT). In 2007 soil samples were collected from the 0–5, 5–10, and 10–20 cm depths, and were separated into four aggregate-size classes (>2 000, 250–2 000, 53–250, and >53 μm) by wet-sieving method. Aggregate-associated C was determined, and the relationships between total soil C concentration and aggregation-size fractions were examined. The results showed that NT and RT treatments significantly increased the proportion of macroaggregate fractions (>2 000 and 250–2 000 μm) compared with the MP-R and MP+R treatments. Averaged across all depths, mean weight diameters of aggregates (MWD) in NT and RT were 47 and 20% higher than that in MP+R. The concentration of bulk soil organic C was positively correlated with MWD ( r =0.98; P =0.024) and macroaggregate fraction ( r =0.96; P =0.036) in the 0–5 cm depth. In the 0–20 cm depth, comparing with MP+R, total C occluded in the >2 000 μm fraction was increased by 9 and 6% under NT and RT, respectively. We conclude that adoption of conservation tillage system, especially no-till, can increase soil macro-aggregation and total C accumulation in macroaggregates, which may improve soil C sequestration in the intensive agricultural region of the North China Plain.

No-Till Farming

Seedbed refers to “the physical state of the surface soil which affects the germination and emergence of crop seeds,” while tilth is “the physical condition of soil as related to its ease of tillage, fitness as a seedbed, and its impedance to seedling emergence and root penetration.” (SSSA, 2008). The concept of soil tilth is still evolving. Current definitions of soil tilth are somewhat subjective and qualitative because of the highly dynamic nature and complexity of the soil. Soil tilth is the product of complex interactive processes varying over space and time. In this Chapter, soil tilth is defined as the physical condition of a soil described by its complex and dynamic macro- and micro-scale physical, hydrological, thermal, chemical, and biological attributes affecting tillage, seedling emergence, root penetration, and plant growth.

Does inorganic nitrogen fertilization improve soil aggregation? Insights from two long-term tillage experiments.

The relationship between inorganic fertilization and soil aggregation is not well understood. We studied cumulative nitrogen (N) fertilization impacts on aggregation, soil organic C (SOC), pH, and their relationships under irrigated and rainfed experiments in Nebraska after 27 and 28 yr, respectively. The dominant soil series were Crete silt loam at the irrigated site, and Coleridge silty clay loam at the rainfed site. We studied irrigated continuous corn ( L.) in chisel plow (CP) and ridge till (RidgeT) receiving 0, 75, 150, and 300 kg N ha yr and rainfed continuous corn and corn-soybean [ (L.) Merr.] in moldboard plow (MP), reduced till (RT), and no-till (NT) with corn receiving 0, 80, and 160 kg N ha yr. Fertilization altered soil aggregation in all tillage systems under continuous corn. Mean weight diameter of water-stable aggregates (MWDA) increased in the upper 7.5-cm depth in NT but decreased in the 7.5- to 60-cm depth by 1.5 times with N application. Fertilization reduced pH but had little or no effect on SOC. Both MWDA and pH ( = 0.47***) decreased under irrigated corn, particularly in the 7.5- to 30-cm depth. No-till and RT had two to five times greater near-surface MWDA than MP. Continuous corn had greater MWDA than corn-soybean in the upper 30-cm depth except in MP. Long-term N fertilization improves near-surface soil aggregation in NT continuous corn but reduces aggregation in the subsoil. Results also suggest that, if fertilizers are applied at rates of about 80 kg N ha, deterioration of soil aggregation would be minimal.

Implications of inorganic fertilization of irrigated corn on soil properties: lessons learned after 50 years.

Inorganic fertilizers are widely used for crop production, but their long-term impacts on soil organic carbon (SOC) pools and soil physical attributes are not fully understood. We studied how half a century of N application at 0, 45, 90, 134, 179, and 224 kg ha and P application at 0, 20, and 40 kg ha (since 1992) affected SOC pools and soil structural and hydraulic parameters in irrigated continuous corn ( L.) under conventional till on an Aridic Haplustoll in the central Great Plains. Application of 45, 90, 134, 179, and 224 kg N ha increased the SOC pool by 4.6, 6.8, 7.6, 7.9, and 9.7 Mg ha, respectively, relative to nonfertilized plots in the 0- to 45-cm depth. Application of 20 kg P ha increased the SOC pool by 2.9 Mg ha in the 0- to 30-cm depth. The highest N rate increased the SOC pool by 195 kg ha yr. The C gains may be, however, offset by the C hidden costs of N fertilization. Application of >45 kg N ha reduced the proportion of soil macroaggregates (>0.25 mm) in the 7.5- to 30-cm depth. Fertilization did not affect hydraulic properties, but application of ≥90 kg N ha slightly increased aggregate water repellency. An increase in SOC concentration did not increase the mean weight diameter of wet aggregates ( = 0.1; > 0.10), but it slightly increased aggregate water repellency ( = 0.5; 0.005). Overall, long-term inorganic fertilization to irrigated corn can increase SOC pool, but it may reduce soil structural stability.