紫云英与化肥配施对安徽沿江双季稻区土壤生物学特性的影响

【目的】紫云英-水稻轮作生产体系是近年来为解决大面积冬闲田而提出的水稻生产新模式。研究紫云英-水稻轮作模式下,不同量紫云英与化肥配施对稻谷增产效果及稻田土壤生物学特性的影响,为合理施用紫云英,有效改善稻田土壤微生态环境、提高土壤质量、保证水稻高产稳产提供理论支撑。【方法】以安徽省农科院土壤肥料研究所2008年设置的紫云英-稻-稻定位试验为平台,分析了5种不同施肥模式下[不施紫云英和化肥(CK)、100%化肥不施紫云英以及70%化肥分别配施紫云英7500、15000、30000 kg/hm2])稻田耕层土壤(0—20 cm)微生物量碳、微生物量氮(SMBC、SMBN)和土壤酶活性的变化,及土壤生物学特性与土壤养分的相关性,并以水稻农艺性状和产量检验了土壤生产力。【结果】1)施用紫云英绿肥能够显著提高水稻籽粒产量,增加水稻单位面积的有效穗数和水稻结实率。尤其是70%化肥配施紫云英15000 kg/hm2处理,稻谷产量达7604.53 kg/hm2,比未施肥处理和单施化肥处理分别增产228.06%和36.29%,差异达显著水平。2)与对照相比,单施化肥处理土壤微生物量碳、氮增加,脲酶、酸性磷酸酶活性提高,过氧化氢酶活性降低;70%化肥的条件下,配施紫云英处理土壤微生物量及土壤酶活性显著高于单施化肥及对照处理,且随紫云英施用量的提高而增加。整个生育期,与对照相比,施紫云英处理土壤微生物量碳、氮分别提高21.03%~142.33%、19.97%~83.91%,土壤脲酶、酸性磷酸酶、过氧化氢酶活性分别提高10.12%~100.33%、10.22%~43.23%、0.14%~7.28%。土壤微生物量及脲酶、酸性磷酸酶与土壤有机质、全氮、碱解氮呈显著或极显著正相关;过氧化氢酶与土壤养分之间无明显相关性。【结论】紫云英绿肥与化肥长期配合施用可显著提高水稻产量、土壤微生物量及土壤酶活性,改善稻田土壤的微生态环境。本试验条件下,在70%化肥施用量的前提下,紫云英施用量以15000~30000 kg/hm2的综合效果较好。故适量紫云英与化肥配施有利于提高土壤生产能力,是安徽沿江双季稻区稻田增产和培肥地力的有效途径。     

Compared effects of long-term pig slurry applications and mineral fertilization on soil denitrification and its end products (N2O, N2)

The long-term (9 years) effect of pig slurry applications vs mineral fertilization on denitrifying activity, N₂O production and soil organic carbon (C) (extractable C, microbial biomass C and total organic C) was compared at three soil depths of adjacent plots. The denitrifying activities were measured on undisturbed soil cores and on sieved soil samples with acetylene method to estimate denitrification rates under field or potential conditions. Pig slurry applications had a moderate impact on the C pools. Total organic C was increased by +6.5% and microbial biomass C by ≥25%. The potential denitrifying activity on soil suspension was stimulated (×1.8, P<0.05) 12 days after the last slurry application. This stimulation was still apparent, but not significant, 10 months later and, according to both methods of denitrifying activity measurement (r ²=0.916, P<0.01 on sieved soil; r ²=0.845, P<0.001 on soil cores), was associated with an increase in microbial biomass C above a threshold of about 105 mg kg⁻¹. The effect of pig slurry on denitrification and N₂O reduction rates was detected on the surface layer (0–20 cm) only. However, no pig slurry effect could be detected on soil cores at field conditions or after NO₃ ⁻ enrichments at 20°C. Although the potential denitrifying activity in sieved soil samples was stimulated, the N₂O production was lower (P<0.03) in the plot fertilized with pig slurry, indicating a lower N₂O/(N₂O + N₂) ratio of the released gases. The pig-slurry-fertilized plot also showed a higher N₂O reduction activity, which is coherent with the lower N₂O production in anaerobiosis.     

Microbial immobilisation of C rhizodeposits in rhizosphere and root-free soil under continuous C labelling of oats

A greenhouse experiment was conducted by growing oats (Avenasativa L.) in a continuously ¹³CO₂ labeled atmosphere. The allocation of ¹³C-labeled photosynthates in plants, microbial biomass in rhizosphere and root-free soil, pools of soil organic C, and CO₂ emissions were examined over the plant's life cycle. To isolate rhizosphere from root-free soil, plant seedlings were placed into bags made of nylon monofilament screen tissue (16 μm mesh) filled with soil. Two peaks of ¹³C in rhizosphere pools of microbial biomass and dissolved organic carbon (DOC), as well as in CO₂ emissions at the earing and ripeness stages were revealed. These ¹³C maxima corresponded to: (i) the end of rapid root growth and (ii) beginning of root decomposition, respectively. The δ¹³C values of microbial biomass were higher than those of DOC and of soil organic matter (SOM). The microbial biomass C accounted for up to 56 and 39% of ¹³C recovered in the rhizosphere and root-free soil, respectively. Between 4 and 28% of ¹³C assimilated was recovered in the root-free soil. Depending on the phenological stage, the contribution of root-derived C to total CO₂ emission from soil varied from 61 to 92% of total CO₂ evolved, including 4-23% attributed to rhizomicrobial respiration. While 81-91% of C substrates used for microbial growth in the root-free soil and rhizosphere came from SOM, the remaining 9-19% of C substrates utilized by the microbial biomass was attributable to rhizodeposition. The use of continuous isotopic labelling and physical separation of root-free and rhizosphere soil, combined with natural ¹³C abundance were effective in gaining new insight on soil and rhizosphere C-cycling.     

Labile carbon and methane uptake as affected by tillage intensity in a Mollisol

Methane (CH₄) oxidation potential of soils decreases with cultivation, but limited information is available regarding the restoration of that capacity with implementation of reduced tillage practices. A study was conducted to assess the impact of tillage intensity on CH₄ oxidation and several C-cycling indices including total and active microbial biomass C (t-MBC, a-MBC), mineralizable C (C_min) and N (N_min), and aggregate-protected C. Intact cores and disturbed soil samples (0–5 and 5–15 cm) were collected from a corn (Zea mays L.)–soybean (Glycine max L. Merr.) rotation under moldboard-plow (MP), chisel-plow (CP) and no-till (NT) for 8 years. An adjacent pasture (<25 years) and secondary growth forest (>60 years) soils were also sampled as references. At all sites, soil was a Kokomo silty clay loam (mesic Typic Argiaquolls). Significant tillage effects on t-MBC and protected C were found in the 0–5 cm depth. Protected C, a measure of C retained within macro-aggregates and defined as the difference in C_min (CO₂ evolved in a 56 days incubation) between intact and sieved (<2 mm) soil samples, amounted to 516, 162 and 121 mg C kg⁻¹ soil in the 0–5 cm layer of the forest, pasture and NT soils, respectively. Protected C was negligible in the CP and MP soils. Methane uptake rate (μg CH₄-C kg⁻¹ soil per day, under ambient CH₄) was higher in forest (2.70) than in pasture (1.22) and cropland (0.61) soils. No significant tillage effect on CH₄ oxidation rate was detected (MP: 0.82; CP: 0.41; NT: 0.61). These results underscore the slow recovery of the CH₄ uptake capacity of soils and suggest that, to have an impact, tillage reduction may need to be implemented for several decades.     

Dryland plant biomass and soil carbon and nitrogen fractions on transient land as influenced by tillage and crop rotation

Soil and crop management practices may alter the quantity, quality, and placement of plant residues that influence soil C and N fractions. We examined the effects of two tillage practices [conventional till (CT) and no-till (NT)] and five crop rotations [continuous spring wheat (Triticum aestivum L.) (CW), spring wheat–fallow (W–F), spring wheat–lentil (Lens culinaris Medic.) (W–L), spring wheat–spring wheat–fallow (W–W–F), and spring wheat–pea (Pisum sativum L.)–fallow (W–P–F)] on transient land previously under 10 years of Conservation Reserve Program (CRP) planting on the amount of plant biomass (stems + leaves) returned to the soil from 1998 to 2003 and soil C and N fractions within the surface 20 cm in March 2004. A continued CRP planting was also included as another treatment for comparing soil C and N fractions. The C and N fractions included soil organic C (SOC), soil total N (STN), microbial biomass C and N (MBC and MBN), potential C and N mineralization (PCM and PNM), and NH₄-N and NO₃-N contents. A field experiment was conducted in a mixture of Scobey clay loam (fine-loamy, mixed, Aridic Argiborolls) and Kevin clay loam (fine, montmorillonitic, Aridic Argiborolls) in Havre, MT, USA. Plant biomass yield varied by crop rotation and year and mean annualized biomass was 45–50% higher in CW and W–F than in W–L. The SOC and PCM were not influenced by treatments. The MBC at 0–5 cm was 26% higher in W–W–F than in W–F. The STN and NO₃-N at 5–20 cm and PNM at 0–5 cm were 17–1206% higher in CT with W–L than in other treatments. Similarly, MBN at 0–5 cm was higher in CT with W–L than in other treatments, except in CT with W–F and W–P–F. Reduction in the length of fallow period increased MBC and MBN but the presence of legumes, such as lentil and pea, in the crop rotation increased soil N fractions. Six years of tillage and crop rotation had minor influence on soil C and N storage between croplands and CRP planting but large differences in active soil C and N fractions.     

Sensitivity analysis of critical parameters in microbial maintenance-energy models

Summary Overestimates of microbial biomass and high maintenance rates have caused calculations of annual maintenance requirements to exceed annual C inputs to soil ecosystems. An integrated approach is needed to resolve this inconsistency in the literature. In the present study a mechanistic model for soil microbial systems was used to calculate the maintenance-energy requirements of the soil microbial biomass. This model is base on product formation rather than substrate use and describes an active and sustaining population, with cryptic growth and necromass recycling. Several assumptions, such as death rates, the percentage of active population, and the yield, are required to calculate the maintenance energies, and the sensitivity of these estimated parameters on the maintenance-rate calculation was tested. The total biomass and the yield factor had the greatest effect on the calculated maintenance value. The fraction of active organisms, the death rates, and the different maintenance values for each population had little effect on the maintenance value.     

Fungal abundance and distribution as influenced by clearing and land use in a vertic soil of Argentina

The influence of native vegetation clearing and different further soil managements on fungal propagule population diversity was studied in the present work. In each of the 3 years (1998, 1999, and 2000), soil samples were collected at the depth of 0–7.5 cm from sites under native vegetation (V0); naturalized prairie, cleared in 1982 (P16); conventional tillage, cleared in 1972 (T26); and direct drilling, cleared in 1958 (D40). Fungal population size and relative abundance of fungal genera were studied by plate counts and further identification of isolates on potato dextrose agar. The undisturbed site and the other sites with increasing time elapsed since native vegetation clearing and different management history showed a distinctive distribution of fungal genera. There were significant differences (p<0.05) among the sites in the abundance of fungal genera analyzed in all the 3 years. Principal component analysis based on relative fungal genus abundance differentiated the sites with 75% variance explained by the first and second components. Diversity and abundance of isolated fungal genera were increased as density of Penicillium spp. decreased, suggesting a competitive effect of this fungal genus. The largest diversity was found in the site under no-till management. The different distribution and relative abundance of the fungal genera studied seemed to be influenced strongly by the management and the presence of surface residue in the no-tilled site.     

Decomposition of pea and maize straw in Pakistani soils along a gradient in salinity

Maize straw and pea straw were added to five Pakistani soils from a gradient in salinity to test the following hypotheses: Increasing salinity at high pH decreases proportionally (1) the decomposition of added straw and (2) the resulting net increase in microbial biomass. In the non-amended control soils, salinity had depressive effects on microbial biomass C, biomass N, but not on biomass P and ergosterol. The ratios microbial biomass C-to-N and biomass C-to-P decreased consistently with increasing salinity. In contrast, the ergosterol-to-microbial biomass C ratio was constant in the four soils at pH>8.9, but nearly doubled in the most saline, but least alkaline, soil (pH 8.2). The addition of the maize and pea straw always increased the contents of microbial biomass C, biomass N, biomass P and ergosterol, but without clear effects of salinity. Highest mean contents of microbial biomass C and biomass N were measured at day 0, immediately after the straw was added. Straw amendments increased the CO₂ evolution rates of all five soils without any effect of salinity. The same was true for total C and total N in the two fractions of particulate organic matter (POM) 63–400 μm and >400 μm. Lowest percentage of straw-derived CO₂-C and highest recoveries of POM-C and POM-N were observed in the maize straw treatment and the reverse in the pea straw treatment. Yield coefficients were calculated for maize and pea straw based on the assumption that the balance gap between CO₂ and the amount of POM can be fully assigned to microbial products.     

Organic matter accumulation and fertilizer-induced acidification interact to affect soil microbial and enzyme activity on a long-term sugarcane management experiment

The effects of crop residue management and fertilizer applications on the size and activity of the microbial community and the activity of exocellular enzymes involved in mineralization of C, N, P and S were examined on a long-term (60 years) field trial under sugarcane situated at Mount Edgecombe, South Africa. Treatments at the site included pre-harvest burning with harvest residues removed (B), burning with harvest residues (unburnt tops) left on the soil surface (B_t) and green cane harvesting with retention of a trash blanket (T). Plots were either fertilized annually with N, P and K or unfertilized. The size and activity of the microbial community and the activity of soil enzymes assayed increased with increasing inputs of crop residues (B < B_t < T) and this effect was evident to a depth of 30 cm. The metabolic quotient was decreased by inputs of both crop residues and fertilizers. Annual fertilizer additions did not affect basal respiration, increased fluorescein diacetate (FDA) hydrolysis rate and acid phosphatase, invertase and protease activities and decreased arginine ammonification rate and dehydrogenase, alkaline phosphatase, arylsulphatase and histidase activities. These effects were attributed to an interaction between the positive effect of fertilizer in increasing the size of the microbial biomass and the negative effect of fertilizer-N-induced soil acidification on microbial activity and on the activity of exocellular enzymes. Such results demonstrate the importance of using a range of measurements of microbial and enzyme activity when determining the effects of management on soil microbial and biochemical properties.