Alan J. Franzluebbers

Soil organic matter stratification ratio as an indicator of soil quality

Soil quality is a concept based on the premise that management can deteriorate, stabilize, or improve soil ecosystem functions. It is hypothesized that the degree of stratification of soil organic C and N pools with soil depth, expressed as a ratio, could indicate soil quality or soil ecosystem functioning, because surface organic matter is essential to erosion control, water infiltration, and conservation of nutrients. Stratification ratios allow a wide diversity of soils to be compared on the same assessment scale because of an internal normalization procedure that accounts for inherent soil differences. Stratification ratios of soil organic C were 1.1, 1.2 and 1.9 under conventional tillage (CT) and 3.4, 2.0 and 2.1 under no tillage (NT) in Georgia, Texas, and Alberta/British Columbia, respectively. The difference in stratification ratio between conventional and NT within an environment was inversely proportional to the standing stock of soil organic C to a depth of 15‐20 cm across environments. Greater stratification of soil C and N pools with the adoption of conservation tillage under inherently low soil organic matter conditions (i.e., warmer climatic regime or coarse-textured soil) suggests that standing stock of soil organic matter alone is a poor indication of soil quality. Stratification of biologically active soil C and N pools (i.e., soil microbial biomass and potential activity) were equally or more sensitive to tillage, cropping intensity, and soil textural variables than stratification of total C and N. High stratification ratios of soil C and N pools could be good indicators of dynamic soil quality, independent of soil type and climatic regime, because ratios >2 would be uncommon under degraded conditions. Published by Elsevier Science B.V.

Soil organic C and N pools under long-term pasture management in the Southern Piedmont USA

Soil organic matter pools under contrasting long-term management systems provide insight into potentials for sequestering soil C, sustaining soil fertility and functioning of the soil‐atmospheric interface. We compared soil C and N pools (total, particulate and microbial) under pastures (1) varying due to harvest technique (grazing or haying), species composition (cool- or warm-season), stand age and previous land use and (2) in comparison with other land uses. Grazed tall fescue-common bermudagrass pasture (20 yr old) had greater soil organic C (31%), total N (34%), particulate organic C (66%), particulate organic N (2.4 fold) and soil microbial biomass C (28%) at a depth of 0‐200 mm than adjacent land in conservation-tillage cropland (24 yr old). Soil organic C and total N at a depth of 0‐200 mm averaged 3800 and 294 g m ˇ2 , respectively, under grazed bermudagrass and 3112 and 219 g m ˇ2 , respectively, under hayed bermudagrass. A chronosequence of grazed tall fescue suggested soil organic N sequestration rates of 7.3, 4.4 and 0.6 g m ˇ2 yr ˇ1 to a depth of 200 mm during 0‐10, 10‐30 and 30‐50 yr, respectively. Soil C storage under long-term grazed tall fescue was 85 to 88% of that under forest, whereas soil N storage was 77 to 90% greater under grazed tall fescue than under forest. Properly grazed pastures in the Southern Piedmont USA have great potential to restore natural soil fertility, sequester soil organic C and N and increase soil biological activity. Published by Elsevier Science Ltd.

Components of surface soil structure under conventional and no-tillage in northwestern Canada

Improvement in soil quality to maintain high production and reduce negative environmental impacts is necessary for alternative crop production strategies to become socially acceptable and viable in the long-term. No-tillage (NT) management of the predominantly small grain region of western Canada has the potential to curb soil erosion and increase profitability. An understanding of the direct effects of NT on soil properties is necessary to evaluate its potential for sustained long-term productivity. We have compiled data collected from two sites in northern British Columbia to ascertain the long-term effects of conventional tillage (CT) and NT on soil components thought to be important in surface soil structural improvement. Soil water retention was greater under NT compared with CT without dramatically altering bulk density due to redistribution of pore size classes into more small pores and less large pores. Soil organic C was greater under NT than under CT nearest the soil surface. Water-stable aggregation improved under NT compared with CT, perhaps because more soil organic C was sequestered within macroaggregates under NT compared with CT that helped to stabilize these aggregates. Steady-state water infiltration was greater under NT than under CT as a result of soil structural improvements associated with surface residue accumulation and lack of soil disturbance. Barley (Hordeum vulgave L.) yield tended to be greater under NT than under CT in years of low rainfall as a result of improvements in soil water retention and transmission that may have provided a better environment for root development. Our data indicate that NT is a viable management strategy to improve soil quality in the cold, semiarid region of western Canada. This strategy could lead to high production, minimal negative environmental impacts, and a socially-acceptable farming system. # 1999 Elsevier Science B.V. All rights reserved.

Potential C and N mineralization and microbial biomass from intact and increasingly disturbed soils of varying texture

Potential C and N mineralization and soil microbial biomass C were determined following disturbance (i.e. drying and sieving) pretreatments in five soils varying in texture (30‐350 mg clay g ˇ1 soil) from the southern Piedmont USA. Soil disturbance by drying (i.e. rewetting following drying at 558C for 72 h) of intact soil cores resulted in a flush of C mineralization (70% to 2.5-fold greater) during 0‐3 d of incubation, but was not significantly diAerent during 3‐10 and 10‐24 d periods compared with field-moist-intact soil cores. Soil disturbance by sieving resulted in greater C mineralization earlier than later in the incubation and led to significant immobilization of N of surface soil where respiration was highest. Increasing soil disturbance through smaller sieve openings resulted in a 10‐60% greater flush of C mineralization that may have been due to disruption of macroaggregates, which protected soil organic C. With a conditioning period of 10 d following rewetting of dried soil, soil microbial biomass C was unaAected by drying or extent of sieving. Soil texture (i.e. clay content) did not interact with disturbance eAects. Immobilization of N was predominant in surface soils (0‐40 mm) of this bermudagrass pasture, where mineralizable C was very high. Carbon mineralization during 0‐3 d was highly related (r 2 =0.9620.04) to C mineralization during 0‐24 d, basal soil respiration and soil microbial biomass C, although increasing soil disturbance (i.e. drying and extent of sieving) altered these relationships in a predictable manner. I conclude that dried and coarsely sieved soil compares favorably to field-moist-intact soil cores for estimating soil microbial biomass and potential activity in landscapes scoured by various degrees of erosion. Published by Elsevier Science Ltd.

Soil-profile distribution of primary and secondary plant-available nutrients under conventional and no tillage

Abstract Nutrient distributions under no tillage (NT) compared with conventional disk-and-bed tillage (CT) management in the warm, humid region of the southeastern USA need to be assessed so that future placement, quantity, and type of fertilizers can be altered, if necessary, to efficiently match crop demands. We determined soil-profile distributions of pH, N, P, S, K, Ca, Mg, Na, Zn, Fe, Mn, and Cu to a depth of 0.9 m at the end of 8.5 years of continuous CT and NT management on a Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) in southcentral Texas. Most dramatic changes occurred within the 0–0.05 m depth, where soil under NT had lower pH, Fe, and Cu than under CT, but greater P, K, Zn, and Mn. Greater P and K under NT than under CT also occurred below the till-zone (0.15–0.3 m). At a depth of 0–0.3 m, soil under NT contained greater amounts of extractable P, K, Zn, Fe, Mn, and Cu than under CT. Nitrogen fertilization had little effect on nutrient distributions, except resulting in greater extractable K at 0–0.05 m and greater nitrate at 0–0.15 m. Few changes in soil-profile distributions were observed for extractable S, Ca, Mg, and Na. Long-term continuous use of NT on this fine-textured, high-fertility (except for N) soil had no apparent adverse effects on nutrient distributions relative to CT, but enhanced conservation and availability of P, K, Zn, Fe, Mn, and Cu near the soil surface where crop roots proliferate.

Active fractions of organic matter in soils with different texture

Summary-Relationships between soil organic C (SOC), soil microbial biomass C (SMBC), mineralizable C and N during a 21 d incubation, and basal soil respiration (BSR) were evaluated on eight soil types from Texas that varied in soil texture (745% clay) and organic matter. The portion of SOC as SMBC increased with increasing clay content, whereas the relationships of mineralizable C and N and BSR to SGC were not affected by soil texture. The ratio of BSR-to-SOC averaged 1.4 + 0.4 mg mineralizable C g-r SOC d-r. The amount of mineralizable C and N and BSR per unit of SMBC, however, decreased with increasing clay content, indicating that the soil microbial biomass (SMB) was more active in coarse-textured soils than in fine-textured soils. The average specific respiratory activit was 29 mg mineralizable C g-’ SMBC d-’ with 10% clay and 11 mg mineralizable C g-’ SMBC d- Y wrth 40% clay. The C-to-N ratio of the mineralizable fraction was 10 f 3 and not affected by soil texture. The established relationships between active soil organic matter (SOM) fractions and soil texture could be used in models predicting SOM turnover. Published by Elsevier Science Ltd

Climatic influences on active fractions of soil organic matter

Biologically active fractions of soil organic matter are important in understanding decomposition potential of organic materials, nutrient cycling dynamics, and biophysical manipulation of soil structure. We evaluated the quantitative relationships among potential C and net N mineralization, soil microbial biomass C (SMBC), and soil organic C (SOC) under four contrasting climatic conditions. Mean SOC values were 28 ^ 11 mg g 21 (na 24) in a frigid‐dry region (Alberta/British Columbia), 25 ^ 5m g g 21 (na 12) in a frigid‐wet region (Maine), 11 ^ 4m g g 21 (na 117) in a thermic‐dry region (Texas), and 12 ^ 5m g g 21 (na 131) in a thermic‐wet region (Georgia). Higher mean annual temperature resulted in consistently greater basal soil respiration (1.7 vs 0.8 mg CO2‐C g 21 SOC d 21 in the thermic compared with the frigid regions, P , 0.001), greater net N mineralization (2.8 vs 1.3 mg inorganic N g 21 SOC 24 d 21 , P , 0.001), and greater SMBC (53 vs 21 mg SMBC g 21 SOC, P , 0.001). Specific respiratory activity of SMBC was, however, consistently lower in the thermic than in the frigid regions (29 vs 34 mg CO2‐C g 21 SMBC d 21 , P , 0.01). Higher mean annual precipitation resulted in consistently lower basal soil respiration (1.1 vs 1.3 mg CO2‐C g 21 SOC d 21 in the wet compared with the dry regions, P , 0.01) and lower SMBC (31 vs 43 mg SMBC g 21 SOC, P , 0.001), but had inconsistent effects on net N mineralization that depended upon temperature regime. Specific respiratory activity of SMBC was consistently greater in the wet than the dry regions ( < 33 vs 29 mg CO2‐C g 21 SMBC d 21 , P , 0.01). Although the thermic regions were not able to retain as high a level of SOC as the frigid regions, due likely to high annual decomposition rates, biologically active soil fractions were as high per mass of soil and even 2‐3-times greater per unit of SOC in the thermic compared with the frigid regions. These results suggest that macroclimate has a large impact on the portion of soil organic matter that is potentially active, but a relatively small impact on the specific respiratory activity of SMBC. Published by Elsevier Science Ltd.

The relationship of land use practices to surface water quality in the Upper Oconee Watershed of Georgia

On a watershed scale, geospatial information can be used to identify water resources that are least buffered from contamination. Implementing conservation practices at these locations may accelerate the process of increasing a watersheds ability to support its designated uses. The Upper Oconee Watershed of Georgia contains land areas devoted to poultry, dairy, and beef production. Within these historically agricultural lands, urbanization is proceeding rapidly around existing cities. Agricultural production practices are concentrated in the watershed with poultry in the headwaters area and dairy near a major lake (Lake Oconee). The objective of this research was to relate data sets representing surface water quality at selected sites throughout the watershed to the predominant land use in that portion of the watershed. The location of 550 poultry operations in the headwaters of the Upper Oconee Watershed, away from the city of Athens GA, has minimized conflicts between agricultural and urban interests. Phosphorus, nitrogen, and fecal coliform bacteria were high near the poultry production area, but were reduced within the watershed prior to reaching the intake for the municipal water supply. Athens had a large impact on surface water quality and approximately doubled the amount of phosphorus and nitrogen in the Oconee River. The Oconee River contributed approximately 70% of the water flowing to Lake Oconee. The residents of Lake Oconee have noted the 30 dairies located west of the lake impacting two relatively minor creeks flowing to the lake. These two creeks make up approximately 2.5% of the flow to the lake, but the proximity of the dairies to the lake makes losses of phosphorus, nitrogen, and fecal bacteria apparent in water samples. Fecal coliform numbers were elevated in some creeks with little agricultural or urban development. To test alternative microbial assays, surface water from a grazed watershed was compared to water from a wooded watershed. Assays for enterococci and E. coli may provide a better test for fecal contamination and allow differentiation between natural areas and areas impacted by grazing animals. Analysis of the Upper Oconee Watershed identified agricultural impacts and areas that should be priorities for natural resource management to reduce agricultural non-point source pollution. Focusing conservation efforts at these locations may prevent agricultural-urban conflict. However, the data also indicate that municipal sources of nutrients and fecal bacteria must be reduced to make significant progress in the watershed.

Bermudagrass Management in the Southern Piedmont USA

al., 1994). Summed to 1-m depth, SOC content was not different between cropland and conservation reserve. Estimates of potential soil organic C (SOC) and total N (TN) In a shortgrass-steppe with 56 yr of grazing in Colorado, sequestration at depths below the traditional plow layer (i.e., 0–0.3 m) in agricultural systems are needed to improve our understanding of SOC was not different between unharvested and lightly management influences on nutrient cycling and potential greenhouse grazed rangeland at any soil depth increment to 0.9-m gas mitigation. We evaluated the factorial combination of nutrient depth(Reederetal.,2004).Incontrast,SOCwasgreater source (inorganic, inorganic cover crop, and broiler litter) and in four of seven depth increments to 0.9-m depth under forage utilization (unharvested, hayed monthly, and low and high heavily grazed compared with unharvested rangeland. grazing pressure) on profile distribution of and changes in SOC and At the end of 12 yr of grazing on a previously ungrazed TN during the first 5 yr of ‘Coastal’ bermudagrass [Cynodon dactylon mixed-grass rangeland in Wyoming, SOC and TN were (L.) Pers.] management. Nutrient source did not affect SOC and TN greater with light and heavy stocking than an ungrazed in the soil profile. Contents of SOC and TN under haying were lower exclosure at a depth of 0 to 0.3 m, but statistically similar than under other management systems throughout the soil profile. between treatments at a depth of 0 to 0.6 m (Schuman et

Particulate and non-particulate fractions of soil organic carbon under pastures in the Southern Piedmont USA.

Pasture management can be effective at sequestering soil organic C. We determined the depth distribution of particulate organic C (POC), non-particulate organic C (NPOC), particulate-to-total organic C (POC-to-TOC) ratio, and particulate organic C-to-N (POC-to-N) ratio under pastures near Watkinsville, GA, USA. POC was highly related with total organic C (TOC), but became an increasingly larger portion of TOC near the soil surface, where both pools were greatest. POC and NPOC were (i) greater under pasture than under conservation-tillage cropland, (ii) greater when pasture was grazed than when hayed, (iii) marginally greater with higher fertilization of pasture, (iv) greater with higher frequency of endophyte infection of tall fescue, and (v) greater under increasing stand age of grass. Soil under pasture comparisons that had greater TOC content had (i) larger improvements in POC than in NPOC and (ii) lower POC-to-N ratios, suggesting improvement in biochemical soil quality, as well as soil C sequestration.