Long‐term effects of cropped vs. fallow and fertilizer amendments on soil organic matter II. Nitrogen

The present study evaluated the effect of fertilizer amendments (organic manure and mineral fertilizers), management practices (fallow and untilled vs. cropped and tilled) on changes of N in bulk soil and N associated with different particle‐size fractions. The long‐term field experiment was conducted since 1962 in Gumpenstein, Austria, on a Dystric Cambisol. The N content of the topsoils changed distinctively during 28 and 38 yr of treatments under both fallow and cropped management practices. Highest increase in total N content was found in animal‐manure (liquid)‐treated plots. The remaining ranking was: animal manure (solid) > cattle slurry > half cattle slurry + straw = PK = NPK. Quite short N‐half‐life values of around 2 yr were found for the cattle‐slurry application, while animal manure exhibited longer N‐half‐lives of around 8 yr. Crop removal of N and mineralization losses in cropped plots obviously were higher than N losses from the bare soil plots lacking a plant cover to keep N in the system. This was confirmed by a consistent shift in the natural ¹⁵N abundances. Comparing the mean N contribution of particle‐size fractions to the total N amounts revealed the following ranking after 28 and 38 yr of different treatments: silt > clay > fine sand > coarse sand, with small exceptions. Particle‐size separates showed more significant responses to changes in the N dynamics of the system due to the various treatments than the bulk soil and can be regarded as the better indicators in this respect.     

Reintroduction of a native Glomus to a tropical Ultisol promoted grain yield in maize after fallow and restored the density of arbuscular mycorrhizal fungal spores

Maize (Zea mays L.) is an important crop in central Thailand where fallow is widely practiced and farmers are interested in crop rotation and beneficial soil biota. A pot experiment using a Typic Paleustult (topsoil + subsoil) from the National Corn and Sorghum Research Centre, Nakhonratchasima Province, Thailand was undertaken over three successive crops to evaluate effects of agronomic practices on populations of arbuscular mycorrhizal (AM) fungi and to determine whether reintroduction of a local Glomus was beneficial to maintain maize yield. The three crops and their treatments were: (1) preceding crop: maize grown in all pots; (2) subexperiment 1: agronomic practices [maize, fallow ± soil disturbance, fallow with solarization, non–AM host (cabbage)]; and (3) subexperiment 2: maize ± Glomus sp. 3 at three rates of P fertilization (0, 33, 92 kg P ha^–1). The AM‐fungal community was established under the preceding crop. In subexperiment 1, the three fallow treatments decreased (30%–40%) the total AM spore number in the topsoil whereas there was no change under maize or cabbage. Glomus, the dominant genus, showed sensitivity to fallow. In subexperiment 2, inoculation with Glomus sp. 3 enhanced total AM spore number and root colonization when applied following the three fallow treatments. Furthermore, inoculation promoted grain yield; at nil P following fallow ± soil disturbance, at 33 kg P ha^–1 following fallow without soil disturbance, and following solarization. Two treatments, maize following maize and maize following cabbage, did not respond to inoculation with Glomus sp. 3. Overall, the results suggest that reintroduction of Glomus sp. 3, a local AM fungus in this soil, may overcome negative effects of fallow and promote effectiveness of P fertilizer. Further work is needed to evaluate the benefits of other indigenous AM species that persist under modern fertilization practices.     

Long‐term effects of soil management regimes on carbon contents and respiration rates of aggregate size fractions

This study investigated long‐term effects of soil management on size distribution of dry‐sieved aggregates in a loess soil together with their organic carbon (OC) and their respiratory activity. Soil management regimes were cropland, which was either abandoned, left bare fallow or cropped for 21 yr. Abandonment increased the abundance of macroaggregates (>2 mm) in the surface soil layer (0–10 cm) and reduced that of microaggregates (<0.25 mm) relative to Cropping, whereas the Fallow treatment reduced the abundance of macroaggregates at depths of 0–10 and 10–20 cm. All treatments yielded similar aggregate size distributions at a depth of 20–30 cm. The SOC content of aggregate size fractions in the surface soil from the Abandoned plots was greater (by 1.2–4.8 g/kg) than that of the corresponding fractions from the Cropped plots, but the opposite trend was observed in the subsurface soils. Conversely, the Fallow treatment reduced the SOC content of every aggregate size fraction. Smaller aggregates generally exhibited greater cumulative levels of C mineralization than larger ones. However, the bulk of the SOC losses from the soils via mineralization was associated with aggregates of >2 mm. Abandonment significantly increased the relative contribution of macroaggregates (>2 mm) to the overall rate of SOC loss, whereas the Fallow treatment significantly reduced the contribution of 0.25–2 mm aggregates to total SOC loss in the surface soil while substantially increasing their contribution in the subsurface soil.     

Quantification of the effects of rotation breaks on soil biological properties and their impact on yield decline in sugarcane

Three contrasting rotation breaks (sown pasture, alternate crops and bare fallow) were established at five sites in Queensland, Australia, on land that had been under sugarcane monoculture for at least 20 years. The breaks were in place for 30-42 months at four sites and for 12 months at the fifth site. The effects of the breaks on selected soil biological properties were assessed following the removal of the breaks and before the area was re-planted with sugarcane. At the four sites with the long-term breaks, microbial biomass increased under the pasture break, declined under the bare fallow break and did not change significantly under the crop break, compared to microbial biomass under continual sugarcane. At these sites, populations of the root lesion nematode (Pratylenchus zeae) declined under all three breaks whereas populations of free-living nematodes increased under the pasture and crop breaks but declined under the bare fallow break. At the site with the 12 month breaks, a forage legume pasture increased microbial biomass, reduced lesion nematodes and together with the crop break increased populations of free-living nematodes. At the four sites with the long-term breaks there was an increase in the ratio of fungal:bacterial fatty acids and an increase in fatty acid 16:1ω5c (used as a biomarker for mycorrhizal fungi) under the pasture and crop breaks. Also at these sites, the soil microbial community under the pasture, crop and bare fallow breaks, respectively, showed increased, no change or diminished capacity to utilize carbon substrates compared to the soil microbial community under continual sugarcane. The yield of the sugarcane crop following all three breaks was significantly higher than the yield of the crop following continual sugarcane at each of the sites with the long-term breaks. Examination of the longevity of the effect of the rotation breaks on soil biological properties at the sites with the long-term breaks, suggested that effects on some soil organisms (e.g. lesion nematodes) maybe short-lived.     

Soil organic matter status of Chernozem soil in North Kazakhstan: effects of summer fallow

In North Kazakhstan there is concern about the degradation of Chernozem soil and agricultural sustainability by the inclusion and frequency of summer fallows in crop rotations in terms of their influence on the changes of soil organic matter (SOM) quality and quantity. We examined five fallow-wheat (Triticum aestivum L.) cropping systems with different frequencies of the fallow phase in Chernozem soil of North Kazakhstan; continuous wheat (CW), 6-y rotation (6R), 4-y rotation (4R), 2-y rotation (2R) and continuous fallow (CF). Soil samples were collected from the two phases of each rotation, pre- and post-fallow, and analyzed for potentially mineralizable C (PMC) and N (PMN), ‘light fraction’ organic matter (LF-OM), C (LF-C) and N (LF-N). Potentially mineralizable C was inversely proportional to the frequency of fallow and was highest in CW. Mineral N (min-N) and PMN were more responsive to rotation phase than other indices of SOM. Mineral N was higher in the post-fallow phase while PMN was higher in the pre-fallow phase. Light fraction organic matter was negatively correlated to the frequency of fallow and was higher in the pre-fallow than in the post-fallow phase in a rotation. The results suggested that the yearly input of plant residue in a less frequently fallowed system built up more PMC, whereas PMN was closely correlated to recent inputs of substrate added with plant residues. We conclude that a frequent fallow system may deplete SOM via accelerated mineralization. Also that LF-OM, PMC and PMN are more sensitive to detect subtle changes in SOM quality than total SOM. Our results may provide prediction of SOM response to fallow frequency in wheat-based rotation systems in Chernozem soils of semi-arid regions.     

Long-term management impacts on soil C, N and physical fertility: Part II: Bad Lauchstadt static and extreme FYM experiments

Manure is a source of plant nutrients and can make a valuable contribution to soil organic matter (SOM). Two experimental sites were studied on a Halpic Phaeozem soil near Bad Lauchstadt in Germany. The first experiment, called the static experiment, commenced in 1902. The impact of fresh farmyard manure (FYM) (0, 20 and 30 t ha⁻¹ 2 year⁻¹) combined with P, K and N fertiliser application on total organic C (C_T), labile C (C_L), non-labile C (C_NL), total N (N_T), mean weight diameter (MWD) and unsaturated hydraulic conductivity (K_unsat) was investigated. The second experiment commenced in 1984 and investigated the effect of extreme rates of fresh FYM applications (0, 50, 100 and 200 t ha⁻¹ year⁻¹) and cropping, or a continuous tilled fallow on the same soil properties. At both sites a nearby grassland site served as a reference. On the static experiment, FYM application increased all C fractions, particularly C_L, where application of 30 t ha⁻¹ 2 year⁻¹ increased C_L by 70% compared with no FYM application. Fertiliser additions to the static experiment had a positive influence on C fractions while N_T increased from both FYM and fertiliser application. MWD increased as a result of FYM application, but did not reach that of the grassland site. Both fertiliser and FYM application increased K_unsat (10 mm tension) on the static experiment. In the second experiment application of 200 t ha⁻¹ year⁻¹ of FYM increased concentrations of C_L by 173% and of C_NL by 80%, compared with no FYM application to make them equivalent to, or greater than the grassland site. A continuously tilled fallow resulted in significant decreases in all C fractions, N_T and MWD compared with the cropped site, while K_unsat (10 mm tension) was increased on the 0 and 50 t ha⁻¹ year⁻¹ treatments as a result of a recent tillage. There was no difference in K_unsat between the cropped and the continuous tilled fallow at FYM applications of 100 and 200 t ha⁻¹ year⁻¹. There were similar significant positive correlations of all C fractions and N_T with MWD on both experimental sites but the relationships were much stronger on the extreme FYM experiment. Weaker relationships of C fractions and N_T with K_unsat (10 mm tension) occurred for the static experimental site but these were not significant for the extreme FYM experimental site. The strongest relationship between C fractions and K_unsat was with C_L. This research has shown that applications of FYM can increase SOM and improve soil physical fertility. However, the potential risk of very high rates of FYM on the environment need to be taken into consideration, especially since the application of organic materials to soils is likely to increase in the future.     

Dynamics of microbial biomass in Cambisols under a three year succession fallow in North Eastern Saxony

The response of microbial biomass carbon (C_mic), nitrogen (N_mic), basal respiration, and the metabolic quotient to 3 years of a natural succession fallow were studied in a field experiment on sandy soil in Northeast Saxony/Germany from 1996 to 1998. Soil samples were taken from Eutric Cambisol and Mollic Cambisol every six weeks during the vegetation period at soil depths of 0—10 and 10—30 cm. The C_mic content in the topsoils increased with time of succession in both soil types. This trend was more distinct in the Mollic Cambisol (70.7 μg g^—1 in June 1996 to 270.9 μg g^—1 in October 1998 at 0—10 cm) than in the Eutric Cambisol (69.7 μg g^—1 in June 1996 to 175.0 μg g^—1 in October 1998 at 0—10 cm). By contrast, the N_mic content slightly decreased in the Eutric Cambisol from 18.9 μg g^—1 to 17.7 μg g^—1 during the same time period. In the Mollic Cambisol, the N_mic increased from 18.8 μg g^—1 in spring 1996 to 35.5 μg g^—1 in fall 1998, however to a lower extent than the C_mic. Subsequently, the (C:N)_mic ratio increased from 4.3 to 5.8 at soil depth of 0—10 cm and from 3.5 to 6.5 at 10—30 cm during the 3‐year‐study at the Eutric Cambisol. In the Mollic Cambisol, the enhancement of (C:N)_mic ratio was more pronounced (i.e. from 4.3 to 6.7 at 0—10 cm and from 3.5 to 7.2 at 10—30 cm). Most likely this results from a shift in microbial populations towards a dominance of soil fungi. The already low basal respiration of, on average, 0.26 mg CO₂ g^—1 (24h)^—1 (0—10 cm) in June 1996 decreased with time of succession fallow to 0.15 and 0.22 mg CO₂ g^—1 (24h)^—1 in October 1998 in the Eutric and the Mollic Cambisol, respectively. Thus, the metabolic quotient as an indicator for the efficiency of organic matter turnover in soil was very low in both soils. During the summer months, the metabolic quotients reached minimum levels of ≤ 0.1 μg CO₂ C (g C_mic)^—1 h^—1, probably because of low soil moisture contents. Correlation analyses revealed close relationships between N_mic and total N, N_mic and water content, and N_mic and pH values. These relationships became even more pronounced with the time period of natural succession. For the samples from fall 1998, highly significant correlations were determined between N_mic and total N (coefficients were r_s = 0.91^***), N_mic and water content (r_s = 0.91^***), and N_mic and pH value (r_s = 0.76^***). The values for all biological parameters studied were larger in the Mollic than in the Eutric Cambisol. This indicates higher turnover rates of different C and N fractions in the Mollic Cambisol. In general, set aside of formerly agricultural managed sandy soils resulted in greater C_mic : N_mic ratios and thus, in a change in the microbiological community structure as well as in reduced C and N turnover rates (i.e. low metabolic quotient) under the climatic conditions of the East German lowlands.     

江西省耕地轮作休耕现状、问题及对策

当前,全国各地正在按照党中央、国务院提出的"探索实行耕地轮作休耕制度试点"的战略部署要求,全力推进探索实行耕地轮作休耕制度试点。为促进江西省及其他条件相近的地区进一步开展相关研究及制定相关政策,以便给实施轮作休耕试点提供参考,通过运用文献研究方法及实地考察等形式,对江西省耕地轮作休耕现状进行了调查,分析存在的问题,并提出了相关对策。结果显示:当前江西耕地作物轮作具有面积比例低、模式多种多样、分布范围广、效益明显等特点,而目前休耕主要采取退耕和休闲两种方式。同时指出当前江西轮作存在面积小、模式差、管理不善、效益低等问题,而休耕多为被动式休耕,存在面积不合理、休耕农田不合适、模式单一、周期无规律、补偿不到位等问题。由以上存在的问题,提出了今后推进全省耕地轮作休耕制度试点的对策与措施:(1)加大宣传力度,提高农民意识;(2)制定科学规划,分类进行;(3)明确范围,有效管理;(4)改善条件,打造样板;(5)确定合理的补偿标准,保障农民利益;(6)增加投入,培养人才。     

Differences in diet between two rodent species,Mastomys natalensis and Gerbilliscus vicinus,in fallow land habitats in central Tanzania

Differences in the ecological niche requirements among rodent species competing in the same habitat may result from differences in the use of one to three resources: space, time and food or some combination of these. Alternatively, differences in resource use utilization among animal species may simply reflect availability of food, and when food is limited, different animal species compete. In this study, the diet of two rodent pest species, Mastomys natalensis and Gerbilliscus vicinus, coexisting in fallow land in central Tanzania were studied to assess the degree of diet differentiation among them. Dietary niche breadth of G. vicinus was greater than that of M. natalensis in all stages of the maize cropping seasons. The rodent species studied overlapped considerably in the food items consumed ranging from niche overlap (O_jk) of 0.77–0.89. Grains/seeds featured high in the diet of M. natalensis while plant material occurrence was high in G. vicinus. These two food categories may have contributed to differences in diet partitioning, which may, in turn, facilitate their coexistence in fallow land.     

黄土区夏闲期土壤呼吸变化特征及其影响因素

本文以1984年设立在黄土旱塬区长期田间定位试验为平台,于2009年采用动态密闭气室法(Li-8100,USA),监测了不同施肥措施下旱地冬麦种植系统中休闲期(7月至9月)土壤呼吸、10cm土层的温度和含水量变化,研究了休闲期土壤呼吸变化特征及其与环境因子的关系。结果表明:黄土旱塬农田休闲期土壤呼吸速率变化剧烈,最大值为5.05μmol m-2s-1,最小值为0.06μmol m-2s-1,平均值为2.00μmol m-2s-1,变异系数为116.5%;整个休闲期不同施肥处理的土壤呼吸速率大小为:化肥有机肥配施处理(NMP)有机肥处理(M)化肥氮磷处理(NP)化肥氮处理(N)和不施肥处理(CK);2009年7月7日至9月11日间NPM、M、NP、N和CK处理土壤CO2-C排放量分别为2.0、1.6、1.2、0.8和0.8 Mg hm-2;土壤呼吸与土壤水分为极显著抛物线关系(p0.01),可解释55%以上的土壤呼吸变异性;土壤呼吸与土壤温度呈显著线性相关(p0.01),但仅能解释呼吸作用变异性的19%~39%;土壤呼吸对耕作的响应强度与微生物量碳极显著线性正相关(p0.01),与土壤有机碳显著线性正相关(p0.05),与全氮、可溶性碳无明显关系(p0.05);降雨对土壤呼吸的促进或抑制主要取决于降雨前的土壤水分状况。长期水分亏缺降雨,降雨可明显促进土壤呼吸,而土壤水分充足时,降雨抑制土壤呼吸,其影响大小与土壤有机碳、全氮、土壤可溶性碳和微生物量碳密切相关。休闲期土壤呼吸受土壤水分、土壤干湿变化、土壤温度、翻耕及土壤有机碳水平等因素的影响。