Soil chemical properties and microbial biomass after 16 years of no-tillage farming on the Loess Plateau, China

Data from a 16-year field experiment conducted in Shanxi, on the Chinese Loess Plateau, were used to compare the long-term effects of no-tillage with straw cover (NTSC) and traditional tillage with straw removal (TTSR) in a winter wheat (Triticum aestivum L.) monoculture. Long-term no-tillage with straw cover increased SOM by 21.7% and TN by 51.0% at 0–10 cm depth and available P by 97.3% at 0–5 cm depth compared to traditional tillage. Soil microbial biomass C and N increased by 135.3% and 104.4% with NTSC compared to TTSR for 0–10 cm depth, respectively. Under NTSC, the metabolic quotient (CO₂ evolved per unit of MBC) decreased by 45.1% on average in the top 10 cm soil layer, which suggests that TTSR produced a microbial pool that was more metabolically active than under NTSC. Consequently, winter wheat yield was about 15.5% higher under NTSC than under TTSR. The data collected from our 16-year experiment show that NTSC is a more sustainable farming system which can improve soil chemical properties, microbial biomass and activity, and thus increase crop yield in the rainfed dryland farming areas of northern China. The soil processes responsible for the improved yields and soil quality, in particular soil organic matter, require further research.     

Organic matter turnover in soil physical fractions following woody plant invasion of grassland: Evidence from natural C and N

Soil physical structure causes differential accessibility of soil organic carbon (SOC) to decomposer organisms and is an important determinant of SOC storage and turnover. Techniques for physical fractionation of soil organic matter in conjunction with isotopic analyses (δ¹³C, δ¹⁵N) of those soil fractions have been used previously to (a) determine where organic C is stored relative to aggregate structure, (b) identify sources of SOC, (c) quantify turnover rates of SOC in specific soil fractions, and (d) evaluate organic matter quality. We used these two complementary approaches to characterize soil C storage and dynamics in the Rio Grande Plains of southern Texas where C₃ trees/shrubs (δ¹³C=−27‰) have largely replaced C₄ grasslands (δ¹³C=−14‰) over the past 100-200 years. Using a chronosequence approach, soils were collected from remnant grasslands (Time 0) and from woody plant stands ranging in age from 10 to 130 years. We separated soil organic matter into specific size/density fractions and determined their C and N concentrations and natural δ¹³C and δ¹⁵N values. Mean residence times (MRTs) of soil fractions were calculated based on changes in their δ¹³C with time after woody encroachment. The shortest MRTs (average=30 years) were associated with all particulate organic matter (POM) fractions not protected within aggregates. Fine POM (53-250 μm) within macro- and microaggregates was relatively more protected from decay, with an average MRT of 60 years. All silt+clay fractions had the longest MRTs (average=360 years) regardless of whether they were found inside or outside of aggregate structure. δ¹⁵N values of soil physical fractions were positively correlated with MRTs of the same fractions, suggesting that higher δ¹⁵N values reflect an increased degree of humification. Increased soil C and N pools in wooded areas were due to both the retention of older C₄-derived organic matter by protection within microaggregates and association with silt+clay, and the accumulation of new C₃-derived organic matter in macroaggregates and POM fractions.     

The effect of tropical forest conversion on soil microbial biomass

We investigated the effects of converting forest to savanna and plough land on the microbial biomass in tropical soils of India. Conversion of the forest led to a significant reduction in soil organic C (40–46%), total N (47–53%), and microbial biomass C (52–58%) in the savanna and the plough land. Among forest, savanna, and plough land, basal soil respiration was maximum in the forest, but the microbial metabolic quotient (qCO₂ was estimated to be at a minimum in the forest and at a maximum in the plough land.     

Effects of liming and tillage systems on microbial biomass and glycosidases in soils

This study was undertaken to investigate the long-term influence of lime application and tillage systems (no-till, ridge-till and chisel plow) on soil microbial biomass C (C_mic) and N (N_mic) and the activities of glycosidases (- and -glucosidases, - and -galactosidases and -glucosaminidase) at their optimal pH values in soils at four agroecosystem sites [Southeast Research Center (SERC), Southwest Research Center (SWRC), Northwest Research Center (NWRC), and Northeast Research Center (NERC)] in Iowa, USA. Results showed that, in general, the C_mic and N_mic values were significantly (P <0.001) and positively correlated with soil pH. Each lime application and tillage system significantly (P <0.001) affected activities of the glycosidases. With the exception of -glucosidase activity, there was no lime×tillage interaction effect. Simple correlation coefficients between the enzyme activities and soil pH values ranged from 0.51 (P <0.05) for the activity of -glucosidase at the NWRC site (surface of the no-till) to 0.98 (P <0.001) at the SWRC site. To assess the sensitivity of the enzymes to changes in soil pH, the linear regression lines were expressed in activity/pH values. In general, their order of sensitivity to changes in soil pH was consistent across the study sites as follow: -glucosidase>-glucosaminidase>-galactosidase>-galactosidase>-glucosidase. Lime application did not significantly affect the specific activities (g p -nitrophenol released kg^–1 soil organic C h^–1) of the enzymes. Among the glycosidases studied, -glucosidase and -glucosaminidase were the most sensitive to soil management practices. Therefore, the activities of these enzymes may provide reliable long-term monitoring tools as early indicators of changes in soil properties induced by liming and tillage systems.     

Microbial biomass and activity at various soil depths in a Brazilian oxisol after two decades of no-tillage and conventional tillage

The advantages of no-tillage (NT) over conventional tillage (CT) systems in improving soil quality are generally accepted, resulting from benefits in soil physical, chemical and biological properties. However, most evaluations have only considered surface soil layers (maximum 0-30 cm depth), and values have not been corrected to account for changes in soil bulk density. The objective of this study was to estimate a more realistic contribution of the NT to soil fertility, by evaluating C- and N-related soil parameters at the 0-60 cm depth in a 20-year experiment established on an oxisol in southern Brazil, with a soybean (summer)/wheat (winter) crop succession under NT and CT. At full flowering of the soybean crop, soil samples were collected at depths of 0-5, 5-10, 10-20, 20-30, 30-40, 40-50 and 50-60 cm. For the overall 0-60 cm layer, correcting the values for soil bulk density, NT significantly increased the stocks of C (18%) and N (16%) and microbial biomass C (35%) and N (23%) (MB-C and -N) in comparison to CT. Microbial basal respiration and microbial quotient (qMic) were also significantly increased under NT. When compared with CT, NT resulted in gains of 0.8 Mg C ha⁻¹ yr⁻¹ (67% of which was in the 0-30 cm layer) and 70 kg N ha⁻¹ yr⁻¹ (73% in the 0-30 cm layer). In the 0-5-cm layer, MB-C was 82% higher with NT than with CT; in addition, the 0-30 cm layer accumulated 70% of the MB-C with NT, and 58% with CT. In comparison to CT, the NT system resulted in total inputs of microbial C and N estimated at 38 kg C ha⁻¹ yr⁻¹ and 1.5 kg N ha⁻¹ yr⁻¹, respectively. Apparently, N was the key nutrient limiting C and N stocks, and since adoption of NT resulted in a significant increase of N in soils which were deficient in N, efforts should be focused on increasing N inputs on NT systems.     

Long-term tillage effects on the distribution patterns of microbial biomass and activities within soil aggregates

The effects of tillage on the interaction between soil structure and microbial biomass vary spatially and temporally for different soil types and cropping systems. We assessed the relationship between soil structure induced by tillage and soil microbial activity at the level of soil aggregates. To this aim, organic C (OC), microbial biomass C (MBC) and soil respiration were measured in water-stable aggregates (WSA) of different sizes from a subtropical rice soil under two tillage systems: conventional tillage (CT) and a combination of ridge with no-tillage (RNT). Soil (0–20 cm) was fractionated into six different aggregate sizes (> 4.76, 4.76–2.0, 2.0–1.0, 1.0–0.25, 0.25–0.053, and < 0.053 mm in diameter). Soil OC, MBC, respiration rate, and metabolic quotient were heterogeneously distributed among soil aggregates while the patterns of aggregate-size distribution were similar among properties, regardless of tillage system. The content of OC within WSA followed the sequence: medium-aggregates (1.0–0.25 mm and 1.0–2.0 mm) > macro-aggregates (4.76–2.0 mm) > micro-aggregates (0.25–0.053 mm) > large aggregates (> 4.76 mm) > silt + clay fractions (< 0.053 mm). The highest levels of MBC were associated with the 1.0–2.0 mm aggregate size class. Significant differences in respiration rates were also observed among different sizes of WSA, and the highest respiration rate was associated with 1.0–2.0 mm aggregates. The C_mic/C_org was greatest for the large-macroaggregates regardless of tillage regimes. This ratio decreased with aggregate size to 1.0–0.25 mm. Soil metabolic quotient (qCO₂) ranged from 3.6 to 17.7 mg CO₂ g^− 1 MBC h^− 1. The distribution pattern of soil microbial biomass and activity was governed by aggregate size, whereas the tillage effect was not significant at the aggregate scale. Tillage regimes that contribute to greater aggregation, such as RNT, also improved soil microbial activity. Soil OC, MBC and respiration rate were at their highest levels for 1.0–2.0 mm aggregates, suggesting a higher biological activity at this aggregate size for the present ecosystem.     

Storage and dynamics of carbon and nitrogen in soil physical fractions following woody plant invasion of grassland

Woody plant invasion of grasslands is prevalent worldwide. In the Rio Grande Plains of Texas, subtropical thorn woodlands dominated by C₃ trees/shrubs have been replacing C₄ grasslands over the past 150 yr, resulting in increased soil organic carbon (SOC) storage and concomitant increases in soil total nitrogen (STN). To elucidate mechanisms of change in SOC and STN, we separated soil organic matter into specific size/density fractions and determined the concentration of C and N in these fractions. Soils were collected from remnant grasslands (Time 0) and woody plant stands (ages 10-130 yr). Rates of whole-soil C and N accrual in the upper 15 cm of the soil profile averaged 10-30 g C m⁻² yr⁻¹ and 1-3 g N m⁻² yr⁻¹, respectively, over the past 130 yr of woodland development. These rates of accumulation have increased soil C and N stocks in older wooded areas by 100-500% relative to remnant grasslands. Probable causes of these increased pool sizes include higher rates of organic matter production in wooded areas, greater inherent biochemical resistance of woody litter to decomposition, and protection of organic matter by stabilization within soil macro- and microaggregates. The mass proportions of the free light fraction (<1.0 g cm⁻³) and macroaggregate fraction (>250 μm) increased linearly with time following woody plant invasion of grassland. Conversely, the mass proportions of free microaggregate (53-250 μm) and free silt+clay (<53 μm) fractions decreased linearly with time after woody invasion, likely reflecting stabilization of these fractions within macroaggregate structures. Carbon and N concentrations increased in all soil fractions with time following woody invasion. Approximately half of the C and N accumulated in free particulate organic matter (POM) fractions, while the remainder accrued in stable macro- and microaggregate structures. Soil C/N ratios indicated that the organic C associated with POM and macroaggregates was of more recent origin (less decomposed) than C associated with the microaggregate and silt+clay fractions. Because grassland-to-woodland conversion has been geographically extensive in grassland ecosystems worldwide during the past century, changes in soil C and N storage and dynamics documented here could have significance for global cycles of those elements.     

长期冬种绿肥改变红壤稻田土壤微生物生物量特性

以进行了31年的双季稻-冬绿肥长期定位试验的红壤性水稻土为对象,研究了长期冬种绿肥条件下土壤微生物生物量特性的季节变化,为阐释冬绿肥措施在稻田生态系统土壤碳氮转化中的作用机制提供理论基础。试验包括双季稻-紫云英(RRV)、双季稻-冬油菜(RRP)、双季稻-黑麦草(RRG)及双季稻-冬闲(RRF)4个处理,在典型时期(绿肥盛花期、绿肥翻压后、早稻成熟期、晚稻收获后)采集土壤样品,分析土壤微生物生物量碳氮及微生物商等方面的变化。结果表明,3种冬绿肥均提高了土壤微生物生物量碳(SMBC)、微生物生物量氮(SMBN)和微生物商,在土壤性质相对稳定的晚稻收获后均显著高于对照处理,全年平均值也多显著高于对照处理。豆科绿肥紫云英对SMBC和SMBN的提高作用最显著,晚稻收获后相对冬闲分别提高了21.12%和98.45%,全年平均值分别提高了15.92%和36.49%。取样时期对SMBC、SMBN和微生物商有较大的影响,但变化趋势基本一致,即绿肥翻压前后无明显变化,早稻成熟期降至最低,至晚稻收获后再次上升。SMBC/SMBN比值在不同绿肥处理间差异不大,但有明显的季节变化,早稻成熟期较高而晚稻收获后降低。因此,南方稻田冬种绿肥后土壤微生物特性明显改善,冬种绿肥是提升稻田土壤肥力的高效措施。     

川西平原土壤微生物生物量碳氮磷含量特征及其影响因素分析

本文通过区域调查采样和统计分析,探讨了川西平原土壤微生物生物量碳（MBC）、土壤微生物生物量氮（MBN）和土壤微生物生物量磷（MBP）含量特征及其对气候、海拔、母质和土地利用等因素的响应,揭示了其关键影响因素,以期为川西平原地区土壤质量管理提供参考。结果表明,不同土壤类型的MBC、MBN和MBP含量表现为冲积土显著高于水稻土、潮土和黄壤（P<0.05）,潮土MBC/MBN显著高于水稻土。气候和海拔的影响为：MBC、MBN和MBP含量随着≥ 0℃积温、≥ 10℃积温、年均温和年均降水量的增加呈指数减少,而随干燥度和海拔增加呈线性增加。不同成土母质中,MBC、MBN和MBP含量均为灰色冲积物显著高于老冲积物。不同土地利用方式下,三者含量为草地显著高于水田和旱地,水田、旱地和林地差异不显著。皮尔森相关分析和冗余分析表明,MBC和MBN均与≥ 0℃积温、年均温呈极显著负相关（P<0.01）,与海拔呈极显著正相关关系,MBP与母质呈现极显著负相关关系。逐步回归分析表明,MBC主要受年均温、干燥度、年均降水量和母质的影响;MBN主要受海拔、干燥度和年均降水量的综合影响;MBP主要受母质、年均温、≥ 10℃积温和年均降水量的调控。因此,川西平原土壤MBC、MBN、MBP能灵敏地反映不同采样点气候的变化,可为该区气候变化下土壤碳、氮、磷的响应预测提供参考。     

宁夏黄土丘陵区植被恢复对土壤养分和微生物生物量的影响

本研究以宁夏固原天然草地、农地、撂荒地和不同年限的柠条林地为研究对象,分析了不同植被类型下土壤养分和土壤微生物生物量碳、氮、磷的变化。研究结果表明:土壤养分除速效磷和全氮外,其他指标均为农地最低,撂荒地次之,并且随植被恢复年限的增加而增加;不同植被类型条件下,土壤微生物生物量有显著差异,微生物量碳含量表现为撂荒地农地天然草地3年柠条林地13年柠条林地23年柠条林地,微生物量氮以天然草地最低,农地、撂荒地和不同年限柠条林地较大,不同植被类型土壤微生物量磷差异显著,在5~20cm土层和20~40cm土壤中表现尤为突出。土壤微生物量碳、氮、磷与植被类型和植被恢复年限关系密切。柠条林对土壤微生物生物量有明显促进作用,并且随着植被恢复年限的增加改良效果越显著。     

Influence of inorganic and organic fertilization on soil microbial biomass, metabolic quotient and heavy metal bioavailability

 We studied the long-term effects (12 years) of municipal refuse compost addition on the total organic carbon (TOC), the amount and activity of the microbial biomass (soil microbial biomass C, B_C and metabolic quotient qCO₂) and heavy metal bioavaiability in soils as compared to manuring with mineral fertilizers (NPK) and farmyard manure (FYM). In addition, we studied the relationships between among the available fraction [Diethylenetriaminopentacetic acid (DTPA) extractable] of heavy metals and their total content, TOC and B_C. After 12 years of repeated treatments, the TOC and B_C of control and mineral fertilized plots did not differ. Soils treated with FYM and composts showed a significant increase in TOC and B_C in response to the increasing amounts of organic C added. Values of the B_C/TOC ratio ranged from 1.4 to 2, without any significative differences among soil treatments. The qCO₂ increased in the organic-amended soil and may have indicated microbial stress. The total amounts of metals in treated soils were lower than the levels permitted by the European Union in agricultural soils. DTPA-extractable metals increased in amended soils in response to organic C. A multiple regression analysis with stepwise selection of variables was carried out in order to discriminate between the influence exerted on DTPA-extractable metals by their total content, TOC and B_C. Results showed that each metal behaved quite differently, suggesting that different mechanisms might be involved in metal bioavailability Received: 31 October 1997     

Soil C balance in a long-term field experiment in relation to the size of the microbial biomass

Soil C balances were calculated in a field experiment started in 1956. Treatments include a fallow and soils receiving different N fertilizers or organic amendments. By assuming the absence of a priming effect, the degree of mineralization of crop residues and organic amendments was calculated. Crop residue mineralization was not affected by a more than 50% decrease in the size of the microbial biomass in soil fertilized with (NH₄)₂SO₄, which had caused the pH of this soil to drop from 6.6 to 4.4. More C had accumulated per unit C input in peat-and sewage sludge-amended soils than in any of the other soils, suggesting that peat and sewage sludge were more resistant to microbial attack. Recalcitrance of substrate C was an adequate explanation for the low ratio of biomass C to soil C in the peat-amended soils, but not in the sewage sludge-amended soil. There was a close linear relationship (r=0.94) between the content of microbial biomass C in the soil measured in 1990 and cumulative C losses from the soil since 1956. Compared to the relationship between soil biomass C and soil organic C concentrations, the linear relationship between microbial C and cumulative C losses suggested that the significantly reduced biomass in the sewage sludge-amended soil was at least partially due to the presence of toxic substances (presumably elevated heavy metal concentrations) in this soil and was probably not affected by the somewhat low pH (5.3) in this soil.     

Change in soil microbial biomass and regulating factors in an alpine meadow site on the Qinghai-Tibetan Plateau

ABSTRACT

The change in soil microbial biomass (SMB) content and its direct links to soil organic matter (SOM) and environmental factors are not well understood for high-elevation regions. Therefore, this research investigated the temporal variation of SMB and its relationship with SOM and environmental factors in an alpine meadow site on the Qinghai-Tibetan Plateau. The soil organic carbon (SOC) and total nitrogen (TN) contents in alpine meadows showed monthly and seasonal variations and were higher in colder months, and the soil C/N ratio was higher in winter and in autumn than it was in the other seasons (P < 0.05). In addition, the changes in the SMB C and SMB N contents were notable at monthly and seasonal scales, whereas the SMB C and SMB N contents were higher in the winter and spring than they were in the other seasons (P < 0.05); the ratio of SMB C/SOC was higher in spring than it was in the other seasons; the ratio of the SMB N/TN content was higher in cold or cool months than it was in the other months; and the ratio of the SMB C/SMB N was highest in August (P < 0.05). Moreover, the SMB C and SMB N contents were significantly positively correlated to the SOC and TN contents (P < 0.01), and the SMB content was also significantly positively correlated to soil water content and air temperature (P < 0.05), or to soil bulk density and soil pH (P < 0.05). The results suggested that the change in the SMB in the alpine meadows was highly regulated by the SOC and TN and by monthly and seasonal changes in the soil bulk density, the soil pH, soil moisture, and air temperatures.     

The spatial distribution of soil microbial biomass in a permanent row crop

The effects of soil texture (silt loam or sandy loam) and cultivation practice (green manure) on the size and spatial distribution of the microbial biomass and its metabolic quotient were investigated in soils planted with a permanent row crop of hops (Humulus lupulus). The soil both between and in the plant rows was sampled at three different depths (0–10, 10–20, and 20–30 cm). The silt loam had a higher overall microbial biomass C concentration (260 g g^-1) than the sandy loam (185 g g^-1), whereas the sandy loam had a higher (3.1 g CO₂-C mg^-1 microbial Ch^-1) metabolic quotient than the silt loam (2.6 g CO₂-C mg^-1 microbial C h^-1), on average over depth (0–30 cm) and over all treatments. There was a sharp decrease in the microbial biomass with increasing depth for all plots. However, this was more pronounced in the silt loam than in the sandy loam. There was no distinct influence of sampling depth on the metabolic quotient. The microbial biomass was considerably higher in the rows than between the rows, especially in the silt loam plots. There was no significant difference between plots without green manure and plots with green manure for either the microbial biomass or the metabolic quotient.     

Influence of polychlorinated biphenyls on microbial biomass and its activity in grassland soil

Microbial biomass content, soil respiration and biomass specific respiration rate were measured in two parts of an area polluted by a municipal waste incinerator [polychlorinated biphenyls (PCBs) from combustion processes]. The soils in the studied parts differed significantly only in their levels of PCBs. The concentration of PCBs found in a control plot (4.4 ng g^-1 soil) can be regarded as a background value while the polluted plot contained an increased amount of PCBs (14.0 ng g^-1 soil). A significantly lower microbial biomass (decreased by 23%, based on the chloroform-fumigation extraction technique) and a lower specific respiration rate (decreased by 14%) were observed in the polluted plot in comparison with the control plot at the end of experimental period (1992–1994). Furthermore, a lower ability of microorganisms in the polluted plot to convert available C_org into new biomass was found in laboratory incubations with glucose-amended samples.     

不同农田生态系统土壤微生物生物量碳的变化研究

试验研究不同农田生态系统土壤微生物生物量碳的变化结果表明,长期单施N、P肥处理对土壤有机碳和微生物生物量碳的影响不明显,施有机肥处理土壤微生物生物量碳及微生物生物量碳/有机碳值均高于其他施肥处理,轮作中引入豆科作物或豆科连作均对土壤微生物生物量碳的积累有显著作用。     

环境因子对山西太岳山土壤微生物量的影响

本文以山西太岳山两个林型(华北落叶松人工林、灌木林)下阴坡和阳坡的土壤样品为研究对象,研究了不同坡向下土壤微生物量碳、氮、土壤理化性质、土壤微生物碳氮比值的变化特征。研究结果表明:土壤有机碳、全氮、微生物量碳、氮含量在0~10 cm土层均显著高于10~20 cm土层。在华北落叶松人工林和灌木林下,土壤微生物量碳、可溶性有机碳及两者之和在阳坡均显著高于阴坡;土壤微生物量碳占两者之和的比例约为81%~87%,在不同坡向下没有显著性差异。土壤微生物量碳氮比在灌木林阳坡下为6.5~7.2,阴坡为7.3~9.9;在华北落叶松人工林阳坡下为7.3~8.6,阴坡为8.2~8.6。两种林型下土壤微生物熵在阳坡均高于阴坡。相关性分析结果表明,土壤微生物量碳、氮与土壤pH值、可溶性有机碳呈显著正相关,土壤有机碳与土壤全氮、微生物量碳、pH值和含水率均有显著正相关性。总的来说,不同坡向对土壤微生物量产生重要影响,进而使土壤微生物熵不同,阴坡土壤有机碳活性较小,碳库的稳定性较好。本文从土壤微生物量及其与环境因子相互关系的角度探讨了土壤环境对土壤微生物的影响,以期为更好的理解该地区土壤碳周转和植被恢复生态效应提供参考。     

Soil microbial biomass C:N:P stoichiometry and microbial use of organic phosphorus

Microbial mineralization and immobilization of nutrients strongly influence soil fertility. We studied microbial biomass stoichiometry, microbial community composition, and microbial use of carbon (C) and phosphorus (P) derived from glucose-6-phosphate in the A and B horizons of two temperate Cambisols with contrasting P availability. In a first incubation experiment, C, nitrogen (N) and P were added to the soils in a full factorial design. Microbial biomass C, N and P concentrations were analyzed by the fumigation-extraction method and microbial community composition was analyzed by a community fingerprinting method (automated ribosomal intergenic spacer analysis, ARISA). In a second experiment, we compared microbial use of C and P from glucose-6-phosphate by adding ¹⁴C or ³³P labeled glucose-6-phosphate to soil. In the first incubation experiment, the microbial biomass increased up to 30-fold due to addition of C, indicating that microbial growth was mainly C limited. Microbial biomass C:N:P stoichiometry changed more strongly due to element addition in the P-poor soils, than in the P-rich soils. The microbial community composition analysis showed that element additions led to stronger changes in the microbial community in the P-poor than in the P-rich soils. Therefore, the changed microbial biomass stoichiometry in the P-poor soils was likely caused by a shift in the microbial community composition. The total recovery of ¹⁴C derived from glucose-6-phosphate in the soil microbial biomass and in the respired CO₂ ranged between 28.2 and 37.1% 66 h after addition of the tracer, while the recovery of ³³P in the soil microbial biomass was 1.4–6.1%. This indicates that even in the P-poor soils microorganisms mineralized organic P and took up more C than P from the organic compound. Thus, microbial mineralization of organic P was driven by microbial need for C rather than for P. In conclusion, our experiments showed that (i) the microbial biomass stoichiometry in the P-poor soils was more susceptible to additions of C, N and P than in the P-rich soils and that (ii) even in the P-poor soils, microorganisms were C-limited and the mineralization of organic P was mainly driven by microbial C demand.     

Size and activity of the soil microbial biomass under grass and arable management

 The effects of 5 years of continuous grass/clover (Cont grass/clover) or grass (Cont grass) pasture or 5 years of annual grass under conventional (Ann grass CT) or zero tillage (Ann grass ZT) were compared with that of 5 years of continuous barley (LT arable) on a site which had previously been under arable crops for 11 years. For added comparison, a long-term grass/clover pasture site (LT past) nearby was also sampled. Soil organic C (C_org) content followed the order LT arable=Ann grass CT<Ann grass ZT<Cont grass=Cont grass/clover<LTpast. Trends with treatment for microbial biomass C (C_mic), basal respiration, flourescein diacetate (FDA) hydrolytic activity, arginine ammonification rate and the activities of dehydrogenase, protease, histidase, acid phosphatase and arylsulphatase enzymes were broadly similar to those for C_org. For C_mic, FDA hydrolysis, arginine ammonification and the activities of histidase, acid phosphatase and arylsulphatase, the percentage increase caused by 5 years of continuous pasture (in comparison with LT arable) was 100–180%, which was considerably greater than that for organic C (i.e. 60%). The microbial metabolic quotient (qCO₂) was higher for the two treatments which were mouldboard ploughed annually (LT arable and Ann grass CT) than for the undisturbed sites. At the undisturbed sites, C_org declined markedly with depth (0–15 cm) and there was a similar stratification in the size and activity of C_mic and enzyme activity. The microbial quotient (C_mic/C_org) declined with depth whilst qCO₂ tended to increase, reflecting a decrease in the proportion of readily available substrate with depth. Received: 7 July 1998