Veronica Acosta‐Martínez

The potential excessive nutrient and/or microbial loading from mismanaged land application of organic fertilizers is forcing changes in animal waste management. Currently, it is not clear to what extent different rates of poultry litter impact soil microbial communities, which control nutrient availability, organic matter quality and quantity, and soil degradation potential. From 2002 to 2004, we investigated the microbial community and several enzyme activities in a Vertisol soil (fine, smectitic, thermic, Udic Haplustert) at 0 to 15 cm as affected by different rates of poultry litter application to pasture (0, 6.7, and 13.4 Mg ha⁻¹) and cultivated sites (0, 4.5, 6.7, 9.0, 11.2, and 13.4 Mg ha⁻¹) in Texas, USA. No differences in soil pH (average: 7.9), total N (pasture: 2.01–3.53, cultivated: 1.09–1.98 g kg⁻¹ soil) or organic C (pasture average: 25–26.7, cultivated average: 13.9–16.1 g kg⁻¹ soil) were observed following the first four years of litter application. Microbial biomass carbon (MBC) and nitrogen (MBN) increased at litter rates greater than 6.7 Mg ha⁻¹ (pasture: MBC = >863, MBN = >88 mg kg⁻¹ soil) compared to sites with no applied litter (MBC = 722, MBN = 69 mg kg⁻¹ soil). Enzyme activities of C (β‐glucosidase, α‐galactosidase, β‐glucosaminidase) or N cycling (β‐glucosaminidase) were increased at litter rates greater than 6.7 Mg ha⁻¹ Enzyme activities of P (alkaline phosphatase) and S (arylsulfatase) mineralization showed the same response in pasture, but they were only increased at the highest (9.0, 11.2, and 13.4 Mg ha⁻¹) litter application rates in cultivated sites. According to fatty acid methyl ester (FAME) analysis, the pasture soils experienced shifts to higher bacterial populations at litter rates of 6.7 Mg ha⁻¹, and shifts to higher fungal populations at the highest litter application rates in cultivated sites. While rates greater than 6.7 Mg ha⁻¹ provided rapid enhancement of the soil microbial populations and enzymatic activities, they result in P application in excess of crop needs. Thus, studies will continue to investigate whether litter application at rates below 6.7 Mg ha⁻¹, previously recommended to maintain water quality, will result in similar improved soil microbial and biochemical functioning with continued annual litter application.

Continuous monoculture systems can reduce soil organic matter because of low organic inputs and disturbance from tillage practices. Integrated cotton ( Gossypium hirsutum ) cropping and livestock production systems in West Texas may provide more sustainable alternatives to the traditional continuous cotton system and improve soil quality. Our study was conducted on a Pullman soil (Fine, mixed, thermic Torrertic Paleustolls) after 5 yr as a complete randomized block design (three replications) that compared continuous cotton and an integrated livestock‐crop system with a perennial warm‐season grass pasture ( Bothriochloa bladhii ) paddock and two paddocks (two stages) of a rotation (wheat [ Triticum aestivum ]‐fallow‐rye [ Secale cereale ]‐cotton). Total N (average: 1.0 g kg ⁻¹ soil) remained similar among systems and soil pH was >8.1. Organic C was higher (13.5 g kg ⁻¹ soil) in perennial pasture compared with continuous cotton (9.0 g kg ⁻¹ soil) at 0 to 5 cm. A similar trend was found for the soil aggregate stability. Soil microbial biomass C (C _mic ) was greater in perennial pasture (193 mg kg ⁻¹ soil) and the rotation under rye and cotton (237 mg kg ⁻¹ soil) compared with continuous cotton (124 mg kg ⁻¹ soil) at 0 to 5 cm, and in perennial pasture at 5 to 10 and 10 to 15 cm. Soil microbial biomass N (N _mic ) showed similar trends. Soil enzyme activities were greater in perennial pasture and the rotation (under rye and cotton) than under continuous cotton at 0 to 5 cm. The integrated crop‐livestock system had higher protozoa (20:4ω6c = 1.98%) and fungi (18:3ω9c = 1.30%) than continuous cotton (20:4ω6c = 1.09%; 18:3ω9c = 0.76%). These findings suggest positive differences in soil function and sustainability of the integrated crop–livestock system compared with continuous cotton.

Little is known about the potential of enzyme activities, which are sensitive to soil properties and management, for the characterization of dust properties. Enzyme activities may be among the dust properties key to identifying the soil source of dust. We generated dust (27 and 7 μm) under controlled laboratory conditions from agricultural soils (0–5 cm) with history of continuous cotton (Gossypium hirsutum L.) or cotton rotated with peanut (Arachis hypogaea L.), sorghum [Sorghum bicolor (L.) Moench], rye (Secale cereale L.), or wheat (Triticum aestivum L.) under different water management (irrigated or dryland) and tillage (conservation or conventional) systems. The 27- and 7-μm dust samples showed activities of β-glucosidase, alkaline phosphatase, and arylsulfatase, which are related to cellulose degradation and phosphorus and sulfur mineralization in soil, respectively. Dust samples generated from a loam and sandy clay loam showed higher enzyme activities compared with dust samples from a fine sandy loam. Enzyme activities of dust samples were significantly correlated to the activities of the soil source with r > 0.74 (P < 0.01). The arylsulfatase proteins contents of the soils (0.04–0.65 mg protein kg⁻¹ soil) were lower than values reported for soils from other regions, but still dust contained arylsulfatase protein. The three enzyme activities studied, as a group, separated the dust samples due to the crop rotation or tillage practice history of the soil source. The results indicated that the enzyme activities of dust will aid in providing better characterization of dust properties and expanding our understanding of soil and air quality impacts related to wind erosion. Copyright © 2004. American Society of Agronomy, Crop Science Society of America, Soil Science Society . ASA, CSSA, SSSA

There is limited knowledge of biochemical processes in low carbon content soils of semiarid regions under different land use and management. This study investigated several enzyme activities of C, N, P, and S transformations in semiarid soils with different clay (10–21 %) and sand (59–85 %) contents that were under conservation reserve program (CRP), native rangeland (NR), and cropland (CL) under sunflowers (Eriophyllum ambiguum (Gray)), continuous cotton (Gossypium hirsutum L.), or in rotations with wheat (Triticum aestivum L.) or sorghum (Sorghum bicolor L.) in West Texas, USA. Soils under CRP and NR showed higher total C and N contents than cultivated soils under continuous cotton, but soil pH (6.7–8.4) was not affected by the management or land use studied. The activities of β‐glucosidase, β‐glucosaminidase, arylamidase, acid and alkaline phosphatase, phosphodiesterase, and arylsulfatase (mg product (kg soil)^–1 h^–1) were lower in CL under continuous cotton compared to cotton in rotation with other crops, CRP, and NR. The enzyme activities were also lower when compared to soils from other regions. Linear regression analyses indicated positive correlations between enzyme activities and total C (r values up to 0.96, P < 0.01). There was a positive relationship between enzyme activities and total N, but soil pH showed the opposite trend. Enzyme activities were significantly intercorrelated with r values up to 0.98 (P < 0.001). The specific enzyme activities (mg product (g organic C)^–1) were lower in continuous cotton in comparison to the uncultivated soils (i.e., NR and CRP) reflecting differences in organic matter quantity and quality due to cultivation. Among the enzymes studied, the specific activities of β‐glucosidase and arylamidase showed a more pronounced decrease with increasing soil depth. In general, soils under CRP or wheat‐cotton rotations revealed higher enzyme activities than soils under the common agricultural practice for these regions, i.e., continuous cotton under conventional tillage.