Frank M. Hons

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

Tillage-induced seasonal changes in soil physical properties affecting soil CO2 evolution under intensive cropping

Crop management practices impact soil productivity by altering the soil environment, which in turn affects microbial growth and decomposition processes that transform plant-produced C to soil organic matter (SOM) or CO2. Reduced tillage increases SOM in the long term, but there is limited information on the in situ seasonal changes in soil physical and biological properties that affect SOM dynamics. Our objectives were to: (i) determine the effect of tillage (conventionally disked (CT) and no tillage (NT)) in a sorghum (Sorghum bicolor (L.) Moench.)-wheat (Triticum aestivum L.)/soybean (Glycine max (L.) Merr.) 2-year rotation sequence and a wheat/soybean double-cropping sequence on the seasonal dynamics and soil depth distribution of gravimetric soil water content (SWC), soil temperature, bulk density (BD) and water-filled pore space (WFPS); and (ii) relate soil CO2 evolution to changes in these physical properties. Treatments had been in place for 9 years at the beginning of sampling. The soil was a Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) located in southcentral Texas. Soil CO2 evolution, using the static chamber method with alkali absorption, and physical properties were measured 57 times during a 2-year period. Soil water content with NT was greater than with CT at the 0–50 mm depth during the fallow period of sorghum-wheat/soybean. In contrast to the surface, SWC at the 50–125 mm depth with CT was greater than or equal to that with NT in all crop sequences. Tillage reduced soil BD, especially at the 50–125 mm depth in all crop sequences, but exhibited large seasonal dynamics at the 0–50 mm depth. Greater temporal variation in SWC and BD occurred due to tillage effects. Soil temperature at 50 mm depth at sunrise averaged 1.2°C greater with NT than with CT during May, September and November, perhaps due to reduced heat loss with residue cover. The mean rate of soil CO2 evolution with CT was 1.55, 1.95 and 2.45 g CO2-C m−2 d−1 in sorghum-wheat/soybean, sorghum-wheat/soybean and wheat/soybean, respectively. The corresponding soil CO2 evolution with NT was 9% greater, 12% greater, and not different compared with CT, respectively. Large seasonal differences in soil CO2 evolution occurred with respect to tillage regime. Soil CO2 evolution was highly related to soil temperature, moisture and temperature-moisture interactions. Regression models of soil CO2 evolution on soil temperature, moisture and day of the season explained from 65 to 98% of the temporal variation, depending upon crop sequence, tillage regime and season. Day of the season was related to non-linear residue decomposition and a polynomial function that expressed crop root respiration and microbial respiration due to rhizodeposition. There were significant tillage and crop sequence interactions with soil temperature and moisture, suggesting that C budgets of agroecosystems derived from climatic data alone could be misleading. Conversion from CT to NT increased C sequestration in soil, but soil under NT released the same or more C as CO2, depending upon crop sequence, suggesting that the dynamics of C sequestration/mineralization had changed during the 10-year period.

Tillage and crop effects on seasonal dynamics of soil CO2 evolution, water content, temperature, and bulk density

Crop management practices impact soil productivity by altering the soil environment, which in turn affects microbial growth and decomposition processes that transform plant-produced C to soil organic matter (SOM) or CO2. Long-term reduced tillage increases SOM, but little is known about the seasonal dynamics of soil CO2 evolution as affected by tillage and crop. The objectives were as follows: (1) To determine the effect of tillage (conventionally disked (CT) and no tillage (NT)) in monoculture sorghum (Sorghum bicolor (L.) Moench.), wheat (Triticum aestivum L.), and soybean (Glycine max (L.) Merr.) on the seasonal dynamics and soil depth distribution of gravimetric soil water content (SWC), soil temperature, bulk density (BD), and water-filled pore space (WFPS). (2) To relate soil CO2 evolution to changes in these parameters. The soil was a Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) located in southcentral Texas. Soil CO2 evolution, using the static chamber method with alkali absorption, and physical parameters were measured 57 times during 2 years, Soil water content was greater under NT than under CT during the fallow period in all crops, but not always during the growing season. Soil BD was reduced shortly after tillage, but increased to levels observed under NT during wet and cold intervals in the fallow period and during the growing season in all crops. Soil temperature at sunrise was 0.7–1.6°C greater under NT than under CT during spring and autumn months in all crops. Mean soil CO2 evolution was greater during the growing season than during fallow in all crops. Mean soil CO2 evolution during the fallow period was not different between tillage regimes in sorghum (1.25 g CO2-C m−2 day−1) , but was greater under NT compared with CT in wheat (1.88 vs. 1.65 g CO2-C m−2 day−1) and in soybean ( 1.04 vs. 0.86 g CO2-C m−2 day−1). During the growing season, soil CO2 evolution was greater under NT compared with CT in sorghum (2.04 vs. 1.84 g CO2-C m−2 day−1) and in soybean (2.08 vs. 1.58 g CO2-C m−2 dayt1), but was lower under NT compared with CT in wheat (2.09 vs. 2.52 g CO2-C m−2 day−1). Mean Soil CO2 evolution during both periods was related to soil organic C (SOC) under CT, but not under NT. The combined effects of soil temperature, SWC (or WFPS), temperature-water interactions, days after harvest during the fallow period, and days after planting during the growing season explained 73 to 92% of the seasonal variation in soil CO2 evolution depending upon tillage regime and crop. In situ soil CO2 evolution responded to soil temperature and SWC differently depending upon tillage and crop management, suggesting that C budgets of agroecosystems derived from climatic data alone could be misleading. Conversion from CT to NT increased C sequestration in soil, but soil under NT released the same or more C as CO2 depending on crop during Year 9 and Year 10 after initiation, suggesting that the dynamics of C sequestration/ mineralization had changed during the 10 year period.

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.

Seasonal changes in soil microbial biomass and mineralizable c and n in wheat management systems

Crop management strategies can affect the short-term dynamics of the active C and N pools of soil organic matter (SOM) by altering the timing, placement, quantity, and quality of crop root and residue input, as well as nutrient status and environmental conditions (i.e. soil temperature and water content). Our objectives were to quantify seasonal changes in soil microbial biomass (SMB) and mineralizable C and N in continuous wheat (Triticum aestivum L.), continuous wheat-soybean [Glycine max (L.) Merr.j, and wheat-soybean-sorghum [Sorghum bicolor (L.) Moench.] sequences under conventional tillage (CT) and no tillage (NT) with or without N fertilization. Soil classified as a Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) located in southcentral Texas was sampled shortly after planting, during flowering, and following harvest of wheat. Soil microbial biomass C (SMBC) increased by 18% and mineralizable C increased by 37% from planting to flowering when averaged across crop sequence, tillage, and N fertilization regimes. At harvest, SMBC and mineralizable C had returned to the amount at planting in all crop sequences, except in continuous wheat, in which decomposition proceeded without C input during the long fallow. Mineralizable C was 64, 28, and 15% greater at flowering compared to planting under NT and 45, 38, and 29% greater under CT at depths of 0 to 50, 50 to 125, and 125 to 200mm, respectively. The greater increase in mineralizable C near the surface may be related to the abundance of crop roots, rhizosphere products, and more optimal air-filled porosity. With N fertilization, mineralizable N followed the same seasonal pattern as SMBC and mineralizable C. Without N fertilization, mineralizable N did not change during the growing season, despite increased SMBC and mineralizable C at flowering, indicating greater immobilization of N at flowering. Seasonal inputs of crop roots, rhizosphere products, and crop residues significantly altered SMB and mineralizable C and N of this soil, illustrating the dependence of N dynamics on short-term C inputs. Seasonal changes in the active C and N pools of SOM depended upon (i) crop sequence for the quantity, quality, and frequency of C input, (ii) tillage for the depth distribution of added substrates, and (iii) N fertilization for the quantity and quality of substrates. These seasonal changes can alter N availability and conservation.

Switchgrass cultivars and germplasm for biomass feedstock production in Texas

Abstract Switchgrass ( Panicum virgatum L.) is a warm-season perennial grass indigenous to North America with excellent potential as a bioenergy crop. Our objective was to determine the yield potential and adaptability of switchgrass cultivars and germplasms in diverse Texas environments where the species might be used as a bioenergy crop. We determined the adaptability of several switchgrass cultivars and germplasms at five ecologically different locations (Beeville, College Station, Dallas, Stephenville, and Temple) in Texas in two experiments during 1992 to 1996. Alamo switchgrass was the best adapted commercially available switchgrass cultivar for biomass feedstock production in Texas in these trials with yields of 8 to 20 Mg ha −1 . A single harvest in the fall maximized biomass yield and maintained switchgrass stands. Although very tolerant of moderate or even severe drought, switchgrass failed to yield under chronic extreme drought. At Beeville in 1996, there was no harvestable switchgrass growth because of extreme drought. Upland cultivars from the midwest matured early and did not produce as much biomass as lowland cultivars from the southern U.S. The predominant factor affecting switchgrass productivity in these Texas locations seemed to be rainfall amount. The highest biomass yield at each location generally occurred in years of greatest April to September rainfall. Soil type did not appear to have much influence on biomass production. Soil organic carbon increased from 11.1 to 15. 8 g kg −1 in the upper 30 cm of soil (average of four locations) during 1992 to 1996. These increases in organic carbon indicate a good potential for sequestering carbon through biomass production.

Soil carbon and nitrogen dynamics as affected by long-term tillage and nitrogen fertilization

Abstract Quantifying seasonal dynamics of active soil C and N pools is important for understanding how production systems can be better managed to sustain long-term soil productivity especially in warm subhumid climates. Our objectives were to determine seasonal dynamics of inorganic soil N, potential C and N mineralization, soil microbial biomass C (SMBC), and the metabolic quotient of microbial biomass in continuous corn (Zea mays L.) under conventional (CT), moldboard (MB), chisel (CH), minimum tillage (MT), and no-tillage (NT) with low (45kgNha–1) and high (90kgNha–1) N fertilization. An Orelia sandy clay loam (fine-loamy, mixed, hyperthermic Typic Ochraqualf) in south Texas, United States, was sampled before corn planting in February, during pollination in May, and following harvest in July. Soil inorganic N, SMBC, and potential C and N mineralization were usually highest in soils under NT, whereas these characteristics were consistently lower throughout the growing season in soils receiving MB tillage. Nitrogen fertilization had little effect on soil inorganic N, SMBC, and potential C and N mineralization. The metabolic quotient of microbial biomass exhibited seasonal patterns inverse to that of SMBC. Seasonal changes in SMBC, inorganic N, and mineralizable C and N indicated the dependence of seasonal C and N dynamics on long-term substrate availability from crop residues. Long-term reduced tillage increased soil organic matter (SOM), SMBC, inorganic N, and labile C and N pools as compared with plowed systems and may be more sustainable over the long term. Seasonal changes in active soil C and N pools were affected more by tillage than by N fertilization in this subhumid climate.

Relationships of chloroform fumigation–incubation to soil organic matter pools

Microbial biomass is part of the active pool of soil organic matter that plays focal roles in decomposition of organic materials, nutrient cycling and biophysical manipulation of soil structure. We compared two commonly used variants of the chloroform fumigation‐incubation method in their relationships with other active, passive and total soil C and N pools in soils from Texas, Georgia, Alberta and British Columbia. The relationship of potential C mineralization with chloroform fumigation‐incubation without subtraction of a control was much stronger (r 2 =0.8120.10 among five data sets with a total of 844 observations) than with subtraction of a control (r 2 =0.3020.22). Similarly, the relationship of soil organic C with chloroform fumigation‐incubation without subtraction of a control was better (r 2 =0.8020.13) than with subtraction of a control (r 2 =0.3820.32). Relationships of net N mineralization, flush of N following fumigation‐incubation, flush of CO2-C during the first day following rewetting of dried soil, particulate organic C and N, mean weight diameter of water-stable aggregation and total porosity with chloroform fumigation‐incubation were also better without subtraction of a control than with subtraction of a control. In analyses of data taken from published reports, chloroform fumigation‐incubation without subtraction of a control was better related with active soil C pools than with subtraction of a control. Chloroform fumigation‐ incubation without subtraction of a control, unlike that with subtraction of a control, should be considered a more robust method to determine microbial biomass under a wide range of environmental conditions. # 1999 Elsevier Science Ltd. All rights reserved.

A rapid procedure for estimating nitrogen mineralization in manured soil

Abstract A routine soil testing procedure for soil N mineralization is needed that is rapid and precise. Not accounting for N mineralization can result in the over-application of N, especially in soils with a history of manure application. Our objectives were to compare results from a recently proposed rapid laboratory procedure with: (1) long-term N mineralization under standard laboratory conditions, and (2) actual forage N uptake from soil receiving dairy cattle (Bos taurus) manure in a 2-year field study. The rapid procedure is based on the quantity of CO2-C evolved during 24 h under optimum laboratory conditions following the rewetting of dried soil. Dairy cattle manure was surface applied beginning in 1992 at annual rates of 0, 112, 224, or 448 kg N ha–1 to field plots on a Windthorst fine sandy loam soil (fine, mixed, thermic Udic Paleustalf) near Stephenville, Texas (32°N, 98°W). Results of the one-day CO2 procedure were highly correlated with soil N mineralized from samples collected in March of 1995 (P=0.004) and 1996 (P<0.001) and with forage N uptake (P<0.001) both years of the study. Residual inorganic N in the same soil samples was poorly correlated with soil N mineralization and forage N uptake.