Seasonal changes in surface aggregate stability under different tillage and crops

Temporal changes in the surface aggregate stability of an Oxic Paleustalf under different tillage practices (direct drilled/stubble retained versus conventional cultivated/stubble burnt) and under different crops (wheat (Triticum aestivum L.) versus lupin (Lupinus angustifolius L.)) were monitored at a 10-year-old rotation site in Wagga Wagga, N.S.W., Australia.

Seasonal fluctuations in aggregate stability were observed under all treatments and were greater than the differences detected between the different tillage and cropping treatments. The seasonal variation was significantly related to the soil water content at the time of sampling and the lowest stability occurred during the autumn/winter period. Cropping under direct drilling and stubble retention resulted in significantly higher stability and lower seasonal fluctuations in stability than under conventional tillage and stubble burning. Despite the seasonal fluctuation, water stability over the season of both of the macroaggregate (more than 250 μm) and microaggregate (less than 50 μm) fractions increased significantly.

While the mean (temporal) stability of the different treatments was significantly related to the mean organic carbon content (r = 0.91) and polysaccharide content, the temporal changes were not related to the soil organic carbon content nor the living root length density.

Lupin had a more beneficial effect on promoting macroaggregate stability than wheat under the conventional tillage/stubble burnt treatment but no significant difference was found under the direct drilled/stubble retained treatment.     

Organic matter, microbial biomass and enzyme activity of soils under different crop rotations in the tropics

Soil organic matter level, soil microbial biomass C, ninhydrin-N, C mineralization, and dehydrogenase and alkaline phosphatase activity were studied in soils under different crop rotations for 6 years. Inclusion of a green manure crop of Sesbania aculeata in the rotation improved soil organic matter status and led to an increase in soil microbial biomass, soil enzyme activity and soil respiratory activity. Microbial biomass C increased from 192 mg kg^–1 soil in a pearl millet-wheat-fallow rotation to 256 mg kg^–1 soil in a pearl millet-wheat-green manure rotation. Inclusion of an oilseed crop such as sunflower or mustard led to a decrease in soil microbial biomass, C mineralization and soil enzyme activity. There was a good correlation between microbial biomass C, ninhydrin-N and dehydrogenase activity. The alkaline phosphatase activity of the soil under different crop rotations was little affected. The results indicate the green manuring improved the organic matter status of the soil and soil microbial activity vital for the nutrient turnover and long-term productivity of the soil. Received: 7 January 1996     

The short-term cover crops increase soil labile organic carbon in southeastern Australia

Little information is available about the effects of cover crops on soil labile organic carbon (C), especially in Australia. In this study, two cover crop species, i.e., wheat and Saia oat, were broadcast-seeded in May 2009 and then crop biomass was crimp-rolled onto the soil surface at anthesis in October 2009 in southeastern Australia. Soil and crop residue samples were taken in December 2009 to investigate the short-term effects of cover crops on soil pH, moisture, NH₄⁺–N, NO₃⁻–N, soluble organic C and nitrogen (N), total organic C and N, and C mineralization in comparison with a nil-crop control (CK). The soil is a Chromic Luvisol according to the FAO classification with 48.4 ± 2.2% sand, 19.5 ± 2.1% silt, and 32.1 ± 2.1% clay. An exponential model fitting was employed to assess soil potentially labile organic C (C ₀) and easily decomposable organic C for all treatments based on 46-day incubations. The results showed that crop residue biomass significantly decreased over the course of 2-month decomposition. The cover crop treatments had significantly higher soil pH, soluble organic C and N, cumulative CO₂–C, C ₀, and easily decomposable organic C, but significantly lower NO₃⁻–N than the CK. However, no significant differences were found in soil moisture, NH₄⁺–N, and total organic C and N contents among the treatments. Our results indicated that the short-term cover crops increased soil labile organic C pools, which might have implications for local agricultural ecosystem managements in this region.     

Impact of land-use change on nitrogen mineralization in subalpine grasslands in the Southern Alps

 A field study was conducted to investigate the effect of abandonment of management on net N mineralization (NNM) in subalpine meadows. NNM, soil microbial biomass N (SMBN), fungal biomass and physicochemical characteristics (total C, total N, dissolved organic carbon (DOC) and pH) were determined in surface (0–10 cm) soil from May to October 1997 in an intensively managed and an abandoned meadow at 1770 m a.s.l.. The cumulative NNM was lower in the abandoned area and ranged from 150 to 373 and from 25 to 85 μg N g^–1 soil in the intensively managed and the abandoned areas, respectively. The total organic C increased in the abandoned area, while total N showed no difference between abandoned and managed meadow. SMBN showed no difference between the investigated sites, whereas ergosterol, a measure of fungal biomass, increased significantly at the abandoned site. The cumulative NNM was negatively correlated with total organic C, C : N ratio, DOC and ergosterol content, and positively correlated with the NH₄ ⁺-N content of soil. The decrease in NNM at the abandoned site may be explained by the lower availability of N in substrates characterized by a high C : N ratio which, together with a decrease in pH in the litter layer, may have increased fungal biomass. Received: 8 January 1999     

Interactions between soil organic matter status, cropping history, method of quantification and sample pretreatment and their effects on measured aggregate stability

 The effects of sample pretreatment (field-moist, air-dried or tension rewetted) on aggregate stability measured by wet sieving or turbidimetry were compared for a group of soil samples ranging in organic C content from 20 to 40 g C kg^–1. Concentrations of total N, total and hot-water-extractable carbohydrate and microbial biomass C were linearly related to those of organic C. Aggregate stability measured by wet sieving using air-dried or field-moist samples and that measured by turbidimetry, regardless of sample pretreatment, increased curvilinearly with increasing soil organic C content. However, when tension-rewetted samples were used for wet sieving, aggregate stability was essentially unaffected by soil organic C content. Measurements of aggregate stability (apart from wet sieving using rewetted soils) were closely correlated with one another and with organic C, total and extractable carbohydrate and microbial biomass C content of the soils. The short-term effects of aggregate stability were also studied. Soils from under long-term arable management and those under long-term arable followed by 1 or 3 years under pasture had similar organic C contents, but aggregate stability measured by turbidimetry and by wet sieving using air-dried or field-moist samples increased with increasing years under pasture. Light fraction C, microbial biomass and hot-water-extractable carbohydrate concentrations also increased. It was concluded that both total and labile soil organic C content are important in relation to water-stable aggregation and that the use of tension-rewetted samples to measure stability by wet sieving is unsatisfactory since little separation of values is achieved. Received: 6 January 1999     

Relationship between soil neutral sugar composition and the amount of labile soil organic matter in Andisol treated with bark compost or leaf litter

 The effect of short-term bark compost (Ba) and leaf litter (Li) applications on the labile soil organic matter (SOM) status was investigated. The SOM status studied in this paper includes soil microbial biomass, soil available N, hot water extractable C (HwC) and N (HwN) and soil neutral sugar-C composition. The soil microbial biomass C (MBC) and N (MBN), soil available N, HwC and HwN increased upon application of Ba and Li. No quantitative relationship was observed between application of organic material and MBC, MBN or soil available N. A positive linear correlation was observed between MBN and HwC but not between MBN and soil available N. Among the various soil neutral sugar C, xylose C (Xyl) content in Ba plots showed a remarkable increase but mannose C (Man) did not differ among Fer (fertilizer), Ba or Li plots. Soil neutral sugar C had a positive linear correlation with soil available N, MBN and HwC. The proportion of MBN : TN is positively correlated with the Xyl/Man ratio. The increase in the proportion of MBN in SOM seems to occur with the increase of SOM derived from plant debris. Received: 20 October 1997     

Soil organic phosphorus and microbial community composition as affected by 26 years of different management strategies

Agricultural management can affect soil organic matter chemistry and microbial community structure, but the relationship between the two is not well understood. We investigated the effect of crop rotation, tillage and stubble management on forms of soil phosphorus (P) as determined by solution ³¹P nuclear magnetic resonance spectroscopy and microbial community composition using fatty acid methyl ester analysis in a long-term field experiment (26 years) on a Chromic Luvisol in New South Wales, Australia. An increase in soil organic carbon, nitrogen and phosphorus compared to the beginning of the experiment was found in a rotation of wheat and subterranean clover with direct drill and mulching, while stubble burning in wheat–lupin and wheat–wheat rotations led to soil organic matter losses. Microbial biomass was highest in the treatment with maximum organic matter contents. The same soil P forms were detected in all samples, but in different amounts. Changes in organic P occurred mainly in the monoester region, with an increase or decrease in peaks that were present also in the sample taken before the beginning of the experiment in 1979. The microbial community composition differed between the five treatments and was affected primarily by crop rotations and to a lesser degree by tillage. A linkage between soil P forms and signature fatty acids was tentatively established, but needs to be verified in further studies.     

Loss of labile organic carbon from subsoil due to land-use changes in subtropical China

Topsoil carbon (C) stocks are known to decrease as a consequence of the conversion of natural ecosystems to plantations or croplands; however, the effect of land use change on subsoil C remains unknown. Here, we hypothesized that the effect of land use change on labile subsoil organic C may be even stronger than for topsoil due to upward concentration of plantations and crops root systems. We evaluated soil labile organic C fractions, including particulate organic carbon (POC) and its components [coarse POC and fine POC], light fraction organic carbon (LFOC), readily oxidizable organic carbon, dissolved organic carbon (DOC) and microbial biomass down to 100 cm soil depth from four typical land use systems in subtropical China. Decrease in fine root biomass was more pronounced below 20 cm than in the overlying topsoil (70% vs. 56% for plantation and 62% vs. 37% for orchard. respectively) driving a reduction in subsoil labile organic C stocks. Land use changes from natural forest to Chinese fir plantation, Chinese chestnut orchard, or sloping tillage reduced soil organic C stocks and that of its labile fractions both in top and subsoil (20–100 cm). POC reduction was mainly driven by a decrease in fine POC in topsoil, while DOC was mainly reduced in subsoil. Fine POC, LFOC and microbial biomass can be useful early indicators of changes in topsoil organic C. In contrast, LFOC and DOC are useful indicators for subsoil. Reduced proportions of fine POC, LFOC, DOC and microbial biomass to soil organic C reflected the decline in soil organic C quality caused by land use changes. We conclude that land use changes decrease C sequestration both in topsoil and subsoil, which is initially indicated by the labile soil organic C fractions.     

Carbon mineralization and microbial activity in a field site trial used for 14C turnover experiments over a period of 30 years

 Long-term experiments on different crop management systems provide essential information about turnover of soil organic matter and changes in microbial properties over a period of time. A long-term field site trial, which was established in 1967 near Vienna, Austria, to document the fate of ¹⁴C-labelled manure (straw and farmyard) under different crop management systems (crop rotation, spring wheat and bare fallow), was investigated. Soil samples were taken in 1997 and separated into size fractions (>250 μm, 250–63 μm, 63–2 μm, 2–0.1 μm and <0.1 μm) after aggregate dispersion using low-energy sonication. Organic C, total N and ¹⁴C content were measured in the bulk soil and the size fractions and microbial properties were analysed in the bulk soil. Additionally, C mineralization in bulk soil samples was monitored at 20 °C over a period of 28 days, and subsequently ¹⁴C-CO₂ content was analysed. The distribution of organic C and N within the size fractions was similar between crop rotation and spring wheat; the highest amounts of organic C and N were found in the clay-sized fraction. The amounts of C and N were significantly smaller in the bare fallow, which was depleted of organic matter in the coarse-sized fractions. ¹⁴C distribution differed significantly from unlabelled C distribution, labelled C was accumulated in the silt-sized fraction, indicating weak humification of the applied manure C. The highest rate of C mineralization was measured in the crop rotation and spring wheat, whereas the emission rate of the bare fallow was about 40% lower. The higher ¹⁴C:C ratio of the bulk soil in comparison to the emitted CO₂ indicated that labelled C compounds still remained mineralizable after a period of 30 years. Microbial properties showed a great difference between crop management systems and bare fallow, particularly regarding urease and xylanase activity. Received: 31 May 1999     

Microbial response of an acid forest soil to experimental soil warming

 Effects of increased soil temperature on soil microbial biomass and dehydrogenase activity were examined on organic (O) horizon material in a low-elevation spruce-fir ecosystem. Soil temperature was maintained at 5  °C above ambient during the growing season in the experimental plots, and soil temperature, moisture, microbial biomass, and dehydrogenase activity were measured during the experiment. An incubation study was also conducted under three temperature regimes, 5, 15, and 25  °C, and under four moisture regimes of 20, 120, 220, and 320% to further evaluate these environmental factors on dehydrogenase activity and microbial biomass. Soil moisture content and microbial biomass controls were significantly lower (30% and 2 μg g^–1 soil, respectively) in the heated plots during the treatment period, suggesting that moisture content was important in controlling microbial biomass. In the incubation study, temperature appeared more important than moisture in controlling microbial biomass and dehydrogenase activity. Increasing temperature between 5  °C and 25  °C resulted in significant decreases in microbial biomass and dehydrogenase activity. Received: 7 August 1998