Effects of carbon and phosphorus addition on microbial respiration,N<Subscript>2</Subscript>O emission,and gross nitrogen mineralization in a phosphorus-limited grassland soil

Soil microbes are frequently limited by carbon (C), but also have a high phosphorus (P) requirement. Little is known about the effect of P availability relative to the availability of C on soil microbial activity. In two separate experiments, we assessed the effect of P addition (20 mg P kg^?1 soil) with and without glucose addition (500 mg C kg^?1 soil) on gross nitrogen (N) mineralization (¹⁵N pool dilution method), microbial respiration, and nitrous oxide (N₂O) emission in a grassland soil. In the first experiment, soils were incubated for 13 days at 90% water holding capacity (WHC) with addition of NO₃^? (99 mg N kg^?1 soil) to support denitrification. Addition of C and P had no effect on gross N mineralization. Initially, N₂O emission significantly increased with glucose, but it decreased at later stages of the incubation, suggesting a shift from C to NO₃^? limitation of denitrifiers. P addition increased the N₂O/CO₂ ratio without glucose but decreased it with glucose addition. Furthermore, the ¹⁵N recovery was lowest with glucose and without P addition, suggesting a glucose by P interaction on the denitrifying community. In the second experiment, soils were incubated for 2 days at 75% WHC without N addition. Glucose addition increased soil ¹⁵N recovery, but had no effect on gross N mineralization. Possibly, glucose addition increased short-term microbial N immobilization, thereby reducing N-substrates for nitrification and denitrification under more aerobic conditions. Our results indicate that both C and P affect N transformations in this grassland soil.     

Nine years of enriched CO2 changes the function and structural diversity of soil microorganisms in a grassland

To gain insight into microbial function following increased atmospheric CO₂ concentration, we investigated the influence of 9 years of enriched CO₂ (600 μl litre⁻¹) on the function and structural diversity of soil microorganisms in a grassland ecosystem under free air carbon dioxide enrichment (FACE), as affected by plant species (Trifolium repens L. and Lolium perenne L. in monocultures and mixed culture) and nitrogen (N) supply. We measured biomass and activities of enzymes covering cycles of the most important elements (C, N and P). The microbial community was profiled by molecular techniques of phospholipid fatty acid (PLFA) and denaturing gradient gel electrophoresis (DGGE) analysis. The enrichment in CO₂ increased soil microbial biomass (+48.1%) as well as activities of invertase (+36.2%), xylanase (+22.9%), urease (+23.8%), protease (+40.2%) and alkaline phosphomonoesterase (+54.1%) in spring 2002. In autumn, the stimulation of microbial biomass was 25% less and that of enzymes 3–12% less than in spring. Strong correlations between activities of invertase, protease, urease and alkaline phosphomonoesterase and microbial biomass were found. The stimulation of microbial activity in the enriched atmosphere was probably caused by changes in the quantity and kind of root litter and rhizodeposition. The response of soil microorganisms to enriched CO₂ was most pronounced under Trifolium monoculture and under greater N supply. The PLFA analysis revealed that total PLFA contents were greater by 24.7% on average, whereby the proportion of bioindicators representative of Gram‐negative bacteria increased significantly in the enriched CO₂ under less N‐fertilized Lolium culture. Discriminant analysis showed marked differences between the PLFA profiles of the three plant communities. Shannon diversity indices calculated from DGGE patterns were greater (+12.5%) in the enriched CO₂, indicating increased soil bacterial diversity. We conclude that greater microbial biomass and enzyme activity buffer the potential increase in C sequestration occurring from greater C addition in enriched CO₂ due to greater mineralization of soil organic matter.     

Dissolved Organic Carbon in Association with Water Soluble Nutrients and Metals in Soils from Lake Okeechobee Watershed, South Florida

Water quality of Lake Okeechobee has been a major environmental concern for many years. Transport of dissolved organic matter (DOM) in runoff water from watershed is critical to the increased inputs of nutrients (N and P) and metals (Cu and Zn). In this study, 124 soil samples were collected with varying soil types, land uses, and soil depths in Lake Okeechobee watershed and analyzed for water-extractable C, N, P, and metals to examine the relationship between dissolved organic carbon (DOC) and water soluble nutrients (N and P) and metals in the soils. DOC in the soils was in 27.64?C400 mg kg^?1 (69.30 mg kg^?1 in average) and varied with soil types, land uses, and soil depth. The highest water-extractable DOC was found in soils collected in sugar cane and field crops (277 and 244 mg kg^?1 in average, respectively). Water soluble concentrations of N and P were in the range of 6.46?C129 and 0.02?C60.79 mg kg^?1, respectively. The ratios of water-extractable C/N and C/P in soils were in 0.68?C12.52 (3.23 in average) and 3.19?C2,329 (216 in average), and varied with land uses. The lowest water-extractable C/N was observed in the soils from dairy (1.66), resident (1.79), and coniferous forest (4.49), whereas the lowest water-extractable C/P was with the land uses of dairy (13.1) and citrus (33.7). Therefore, N and P in the soils under these land uses may have high availability and leaching potential. The concentrations of water soluble Co, Cr, Cu, Ni, and Zn were in the ranges of?<?method detection limit (MDL)?C0.33, <MDL?C0.53, 0.04?C2.42, <MDL?C0.71, and 0.09?C1.13 mg kg^?1, with corresponding mean values of 0.02, 0.01, 0.50, 0.07, and 0.37 mg kg^?1, respectively. The highest water soluble Co (0.10 mg kg^?1), Cr (0.26 mg kg^?1), Ni (0.31 mg kg^?1), and Zn (0.80 mg kg^?1) were observed in soils under the land use of sugar cane, whereas the highest Cu (1.50 mg kg^?1) was with field crop. The concentration of DOC was positively correlated with total organic carbon (TOC) (P <0.01), water soluble N (P <0.01), electrical conductivity (EC, P <0.01), and water soluble Co, Cr, Ni, and Zn (P <0.01), and Cu (P <0.05), whereas water soluble N was positively correlated with water soluble P, Cu, and Zn (P <0.01) in soils. These results indicate that the transport of DOC from land to water bodies may correlate with the loss of macro-nutrients (N, P), micro-nutrients (Cu, Zn, and Ni), and contaminants (Cr and Co) as well.     

Soil moisture, carbon and nitrogen dynamics following incorporation and surface application of labelled crop residues in soil columns

One way to increase the amount of carbon sequestered in agricultural land is to convert conventional tillage into no‐tillage systems. This greatly affects the location of crop residues in soil. To investigate the impact of the location of residues on soil physical and biological properties and how the interactions between those properties influence the fate of carbon and nitrogen in soil, we did a laboratory experiment with repacked soil in columns. Doubly labelled ¹³C¹⁵N oilseed rape residues were incorporated in the 0–10 cm layer or left on the soil surface. The columns were incubated for 9 weeks at 20°C and were submitted to three cycles of drying and wetting, each of them induced by a rain simulator. The location of the residues affected the water dynamics and the distribution of C and N in the soil, which in turn influenced microbial activity and the decomposition rate of the added residues. After 9 weeks of’incubation, 18.4 ± 1.5% of the surface applied residue‐C and 54.7 ± 1.3% of the incorporated residue‐C was mineralized. We observed a nitrate accumulation of 10.7 mg N kg^?1 with residues at the soil surface, 3.6 mg N kg^?1 with incorporated residues and 6.3 mg N kg^?1 without addition of fresh organic matter, which entailed net N mineralization in soil under mulch and immobilization of N with residue incorporation compared with the control soil. We concluded that application of oilseed rape residues at the soil surface increased the storage of fresh organic C in soil in the short term, compared with the incorporation treatment, but increased the risk of nitrate leaching.     

Responses of carbon,nitrogen and phosphorus to two consecutive drying–rewetting cycles in soils

Drying and rewetting cycles are known to be important for the dynamics of carbon (C), phosphorus (P), and nitrogen (N) in soils. This study reports the short‐term responses of these nutrients to consecutive drying and rewetting cycles and how varying soil moisture content affects microbial biomass C and P (MBC and MBP), as well as associated carbon dioxide (CO₂) and nitrous oxide (N₂O) emissions. The soil was incubated for 14 d during which two successive drying–rewetting episodes were imposed on the soils. Soils subjected to drying (DRW) were rewetted on the seventh day of each drying period to return them to 60% water holding capacity, whilst continually moist samples (M), with soil maintained at 60% water holding capacity, were used as control samples. During the first seven days, the DRW samples showed significant increases in extractable ammonium, total oxidized nitrogen, and bicarbonate extractable P concentrations. Rewetting after the first drying event produced significant increases only in CO₂ flux (55.4 µg C g^?1 d^?1). The MBC and MBP concentrations fluctuated throughout the incubation in both treatments and only the second drying–rewetting event resulted in a significantly MBC decrease (416.2 and 366.8 mg kg^?1 in M and DRW soils, respectively). The two drying–rewetting events impacted the microbial biomass, but distinguishing the different impacts of microbial versus physical impacts of the perturbation is difficult. However, this study, having a combined approach (C, N, and P), indicates the importance of understanding how soils will react to changing patterns of drying–rewetting under future climate change.     

Effects of glucose and rhizodeposits (with or without cysteine-S) on immobilized-35S, microbial biomass-35S and arylsulphatase activity in a calcareous and an acid brown soil

Our aim was to study the effects of C (as glucose and artificial rhizodeposits) on S immobilization, in relation to microbial biomass‐S and soil arylsulphatase (ARS) activity, in contrasting soils (a calcareous and an acid brown soil). The glucose‐C and artificial rhizodeposit‐C with or without cysteine were added at six rates (0, 100, 200, 400, 600 and 800 mg kg^?1 soil) to the two soils and then incubated with Na₂³⁵SO₄ for 1 week prior to analysis. The percentages of ³⁵S immobilized increased when C as glucose and rhizodeposit (without cysteine) were added to both soils. With cysteine‐containing rhizodeposit, the percentages of ³⁵S immobilized remained relatively stable (23.5% to 29.9%) in the calcareous soil, but decreased in the acid brown soil (52.7% to 31.5%). For both soils, cysteine‐containing rhizodeposit additions showed no significant correlation between immobilized‐³⁵S and microbial biomass‐³⁵S, suggesting that microorganisms immobilized cysteine‐S preferentially instead of ³⁵S from the tracer (Na₂³⁵SO₄). In the calcareous soil, a positive and significant correlation was found between ARS activity and microbial biomass‐³⁵S (r = 0.85, P < 0.05) when glucose was added. We also saw this correlation in the acid brown soil when rhizodeposit‐C without cysteine was added (r = 0.90, P < 0.05). Accordingly, the results showed the presence of extracellular arylsulphatase activity of 48.7 mg p‐nitrophenol kg^?1 soil hour^?1 in the calcareous soil and of 27.0 mg p‐nitrophenol kg^?1 soil hour^?1 in the acid brown soil.     

Impacts of Smallholder Land Use on Physical-Based Soil Organic Matter Fractions in Soils with Varying Texture in Ecotopes of Eastern Cape Province,South Africa

Physical-based fractions of SOM were examined. Soil carbon (C) and nitrogen (N) across ecotopes were 17.22 g kg^?1 and 3.73 g kg^?1, respectively. Soil C and N were higher in conventional tillage (CT) than no-till (NT) by 2.94% and 0.94%, respectively. Soil C ranged from 11.09 g kg^?1 in silt to 18.02 g kg^?1 in coarse sand; from 12.89 g kg^?1 in fine sand to 18.88 g kg^?1 in clay under NT and CT, respectively. Soil N ranged from 4.54 g kg^?1 in silt to 5.55 g kg^?1 in clay; from 5.06 g kg^?1 in coarse sand to 5.56 g kg^?1 in silt under NT and CT, respectively. Soil N in bulk soil changed by ?3.24% while soil C in bulk soil changed by ?11.87%. The silt + clay was saturated; hence, studies on soil C and N dynamics in these ecotopes are advocated.     

中国土壤微生物量的大小

The microbial biomass C, N and P of soils all over China were determined in this study to study their affecting factors. The results, about 100-417 mg C kg^-1 soil, 18-51 mg N kg^-1 soil and 4.4-27.3 mg P kg^-1 soil, showed the biomass C, N and P in linear relationship with the soil total organic C, toal N and soil organic P. The ratios of C: N and C:P, ranging from 5.6 to 9.6 and from 11.2 to 48.4 respectively, were affected by soil pH, texture, crop rotation, macroclimate, etc. The ratio of C:N in soil biomass increases gradually from the north to the south in China.     

Influence of soil phosphorus status and nitrogen addition on carbon mineralization from 14C-labelled glucose in pasture soils

 This study examines the effect of soil P status and N addition on the decomposition of ¹⁴C-labelled glucose to assess the consequences of reduced fertilizer inputs on the functioning of pastoral systems. A contrast in soil P fertility was obtained by selecting two hill pasture soils with different fertilizer history. At the two selected sites, representing low (LF) and high (HF) fertility status, total P concentrations were 640 and 820 mg kg^–1 and annual pasture production was 4,868 and 14,120 kg DM ha^–1 respectively. Soils were amended with ¹⁴C-labelled glucose (2,076 mg C kg^–1 soil), with and without the addition of N (207 mg kg^–1 soil), and incubated for 168 days. During incubation, the amounts of ¹⁴CO₂ respired, microbial biomass C and ¹⁴C, microbial biomass P, extractable inorganic P (P_i) and net N mineralization were determined periodically. Carbon turnover was greatly influenced by nutrient P availability. The amount of glucose-derived ¹⁴CO₂ production was high (72%) in the HF and low (67%) in the LF soil, as were microbial biomass C and P concentrations. The ¹⁴C that remained in the microbial biomass at the end of the 6-month incubation was higher in the LF soil (15%) than in the HF soil (11%). Fluctuations in P_i in the LF soil during incubation were small compared with those in HF soil, suggesting that P was cycling through microbial biomass. The concentrations of P_i were significantly greater in the HF samples throughout the incubation than in the LF samples. Net N mineralization and nitrification rates were also low in the LF soils, indicating a slow turnover of microorganisms under limited nutrient supply. Addition of N had little effect on biomass ¹⁴C and glucose utilization. This suggests that, at limiting P fertility, C turnover is retarded because microbial biomass becomes less efficient in the utilization of substrates. Received: 18 October 1999     

生物复混肥对土壤微生物群落功能多样性和微生物量的影响

在温室盆栽条件下,采用Biolog微平板法和氯仿熏蒸浸提法,研究了玉米施用等养分量的无机肥、有机无机复混肥和生物复混肥后土壤微生物群落功能多样性及土壤微生物量的变化。结果表明:生物复混肥处理的土壤微生物平均颜色变化率(AWCD)、微生物群落Shannon指数(H)和微生物群落丰富度指数(S)均最高;施用生物复混肥可明显提高土壤微生物对碳源的利用率,尤其是多酚化合物类和糖类;不同处理土壤微生物碳源利用特征有一定差异,生物复混肥在第1主成分上的得分值为正值,其他各处理在第1主成分上的得分值基本上为负值,起分异作用的主要碳源是糖类和羧酸类。在玉米生长期间各处理土壤微生物量大致呈先升高后逐渐平稳的趋势,且土壤微生物量碳、氮、磷的含量均以生物复混肥处理最高,最高值分别为333.21mg.kg 1、53.02 mg.kg 1和22.20 mg.kg 1。研究表明,生物复混肥的施用比等养分量的有机无机复混肥处理能显著提高土壤微生物群落碳源利用率、微生物群落丰富度和功能多样性,显著增加土壤微生物量碳、氮、磷的含量,有利于维持良好的土壤微生态环境。     

Response of soil C:N:P stoichiometry,organic carbon stock,and release to wetland grasslandification in Mu Us Desert

Purpose

Wetlands in Mu Us Desert have severely been threatened by grasslandification over the past decades. Therefore, we studied the impacts of grasslandification on soil carbon (C):nitrogen (N):phosphorus (P) stoichiometry, soil organic carbon (SOC) stock, and release in wetland-grassland transitional zone in Mu Us Desert.

Materials and methods

From wetland to grassland, the transition zone was divided into five different successional stages according to plant communities and soil water conditions. At every stage, soil physical and chemical properties were determined and C:N:P ratios were calculated. SOC stock and soil respirations were also determined to assess soil carbon storage and release.

Results and discussion

After grasslandification, SOC contents of top soils (0–10 cm) decreased from 100.2 to 31.79 g kg^?1 in June and from 103.7 to 32.5 g kg^?1 in October; total nitrogen (TN) contents of top soils (0–10 cm) decreased from 3.65 to 1.85 g kg^?1 in June and from 6.43 to 3.36 g kg^?1 in October; and total phosphorus (TP) contents of top soils (0–10 cm) decreased from 179.4 to 117.4 mg kg^?1 in June and from 368.6 to 227.8 mg kg^?1 in October. From stages Typha angustifolia wetland (TAW) to Phalaris arundinacea L. (PAL), in the top soil (0–10 cm), C:N ratios decreased from 32.2 to 16.9 in June and from 19.0 to 11.8 in October; C:P ratios decreased from 1519.2 to 580.5 in June and from 19.0 to 11.8 in October; and N:P ratios decreased from 46.9 to 34.8 in June and changed from 34.9 to 34.0 in October. SOC stock decreased and soil respiration increased with grasslandification. The decrease of SOC, TN, and TP contents was attributed to the reduction of aboveground biomass and mineralization of SOM, and the decrease of soil C:N, C:P, and N:P ratios was mainly attributed to the faster decreasing speeds of SOC than TN and TP. The reduction of aboveground biomass and increased SOC release led by enhanced soil respiration were the main reasons of SOC stock decrease.

Conclusions

Grasslandification led to lowers levels of SOC, TN, TP, and soil C:N, C:P, and N:P ratios. Grasslandification also led to higher SOC loss, and increased soil respiration was the main reason. Since it is difficult to restore grassland to original wetland, efficient practices should be conducted to reduce water drainage from wetland to prevent grasslandification.

     

中国亚热带稻田土壤碳氮含量及矿化动态

Dynamics of soil organic matter in a cultivation chronosequence of paddy fields were studied in subtropical China. Mineralization of soil organic matter was determined by measuring CO2 evolution from soil during 20 days of laboratory incubation. In the first 30 years of cultivation, soil organic C and N contents increased rapidly. After 30 years, 0-10 cm soil contained 19.6 g kg^-1 organic C and 1.62 g kg^-1 total N, with the corresponding values of 18.1 g kg^-1 and 1.50 g kg^-1 for 10-20 cm, and then remained stable even after 80 years of rice cultivation. During 20 days incubation the mineralization rates of organic C and N in surface soil （0-10 cm） ranged from 2.2% to 3.3% and from 2.8% to 6.7%, respectively, of organic C and total N contents. Biologically active C size generally increased with increasing soil organic C and N contents. Soil dissolved organic C decreased after cultivation of wasteland to 10 years paddy field and then increased. Soil microbial biomass C increased with number of years under cultivation, while soil microbial biomass N increased during the first 30 years of cultivation and then stabilized. After 30 years of cultivation surface soil （0-10 cm） contained 332.8 mg kg^-1 of microbial biomass C and 23.85 mg kg^-1 of microbial biomass N, which were 111% and 47% higher than those in soil cultivated for 3 years. It was suggested that surface soil with 30 years of rice cultivation in subtropical China would have attained a steady state of organic C content, being about 19 g kg^-1.     

Ecotoxicological effects of fluorine deposits on microbial biomass and enzyme activity in grassland

The influence of atmospheric fluoride deposits on the soil microbial biomass and its enzyme activities was investigated near the aluminium smelter at Ranshofen, Upper Austria. Soil samples at various distances from the emission source were analyzed for water-extractable F and microbial activity. The water-extractable F contents at the sites examined reflected the gradient of F exposure (10 to 189 mg F kg^?l dry soil). The microbial activities increased with distance from the emission source and were inversely correlated with the degree of F contamination. The linear correlation coefficients between the water-extractable F concentrations and the microbial biomass, dehydrogenase and arylsulphatase activity were r=?0·8, ?0·86 and ?0·84, respectively. In the most contaminated soil (up to 189 mg F kg^?1), the microbial activities were only 5–20% of those in the unpolluted soil. The microbial biomass and dehydrogenase activity decreased substantially where the concentration of F exceeded 100mgkg^?1, whereas arylsulphatase activity was already inhibited at 20 mg kg^?1. The accumulation of organic matter near the smelter (123 mg F kg^?1) also indicated severe inhibition of the microbial activity by F. Our investigations show that the ratio of arylsulphatase to microbial biomass can be used as a sensitive index for evaluating environmental stress such as F Contamination.     

The effect of a perennial cover crop on net soil N mineralization and microbial biomass carbon in coconut plantations in the humid tropics

The decrease in soil fertility in agroecosystems due to continuous harvesting, loss of fine soil and oxidation of soil organic carbon (SOC) is well known. This study evaluates for a humid tropical climate in South Andaman Island, India, the impact of a 15-yr old Pueraria cover crop (CC) under a coconut plantation, with and without phosphorus (P) application, on the soil N mineralization rate (NMR), the mineral N pool (NH₄⁺-N and NO₃⁻-N), microbial biomass carbon (MB-C) and SM under four treatments, (1) no cover crop (NCC), (2) no cover crop + phosphorus (NCC+P), (3) CC, and (4) cover crop + phosphorus (CC+P) during three seasons, wet (May–October), post-wet (November–January) and dry (February–April). The NCC treatment served as a control. In addition, an ex-situ experiment was conducted to verify the effect of P application on NMR and MB-C under 100% field capacity (FC), 50 and 25% FC representing the different seasons. The NMR, mineral N pool and MB-C increased by 37, 46 and 41%, respectively under the CC compared to the control. SOC and fine soil particles were also greater under the CC by 41 and 461%, respectively, compared to the control. The application of P to the CC increased soil N mineralization, the mineral N pool and MB-C by 33, 16 and 14%, respectively. The amount of mineralized N was greater under the CC and CC+P treatments by 39 and 73%, respectively than the control. The ex-situ experiment showed that the P application increased NMR and MB-C, but the increases were highest in the 50% FC and lowest in 100% FC. It is proposed for the humid tropics that a CC could be used for enhancing SOC and increasing soil N mineralization under coconut plantations and other similar agroecosystems.     

Analysis of Citrus Orchard Efficiency in Relation to Soil Properties

ABSTRACT

Orchard efficiency (OE) is one of the indices of evaluating the sustainability in production behavior of citrus orchards. A wide range of soil properties broadly categorized into particle size distribution, water soluble and exchangeable cations, and soil available nutrients were investigated in relation to efficiency of Nagpur mandarin (Citrus reticulata Blanco) orchards established on smectite rich three soil orders (Entisols, Inceptisols, and Vertisols) representing 18 locations of central India. The soil properties, viz., free calcium carbonate (CaCO₃), clay content, water soluble- and exchangeable-calcium (Ca^{2 +}), available nitrogen (N), phosphorus (P), and zinc (Zn) contributed significantly towards variation in OE. The threshold limit of these limiting soil properties was further established using multivariate quadratic regression models as: 132.1 g kg^{? 1} free CaCO₃, 418.1 g kg^{? 1} clay, 149.9 mg L^{? 1} water soluble Ca^{2 +}, 25.9 cmol(p⁺) kg^{? 1} exchangeable Ca^{2 +}, 114.6 mg kg^{? 1} available N, 12.8 mg kg^{? 1} available P, and 0.96 mg kg^{? 1} available Zn in relation to optimum OE of 82.1%. These reference values were very close to those obtained from best fit models, and could be effectively utilized in addressing soil related production constraints for precision-aided citriculture.     

Labile organic matter fractions as early‐season nitrogen supply indicators in manure‐amended soils

Soil test indicators are needed to predict the contribution of soil organic N to crop N requirements. Labile organic matter (OM) fractions containing C and N are readily metabolized by soil microorganisms, which leads to N mineralization and contributes to the soil N supply to crops. The objective of this study was to identify labile OM fractions that could be indicators of the soil N supply by evaluating the relationship between the soil N supply, the C and N concentrations, and C/N ratios of water extractable OM, hot‐water extractable OM, particulate OM, microbial biomass, and salt extractable OM. Labile OM fractions were measured before planting spring wheat (Triticum aestivum L.) in fertilized soils and the soil N supply was determined from the wheat N uptake and soil mineral N concentration after 6 weeks. Prior to the study, fertilized sandy loam and silty clay soils received three annual applications of 90 kg available N (ha · y)^?1 from mineral fertilizer, liquid dairy cattle manure, liquid swine manure or solid poultry litter, and there was a zero‐N control. Water extractable organic N was the only labile OM fraction to be affected by fertilization in both soil types (P < 0.01). Across both test soils, the soil N supply was significantly correlated with the particulate OM N (r = 0.87, P < 0.001), the particulate OM C (r = 0.83, P < 0.001), and hot‐water extractable organic N (r = 0.81, P < 0.001). We conclude that pre‐planting concentrations of particulate OM and hot‐water extractable organic N could be early season indicators of the soil N supply in fertilized soils of the Saint Lawrence River Lowlands in Quebec, Canada. The suitability of these pre‐planting indicators to predict the soil N supply under field conditions and in fertilized soils from other regions remains to be determined.     

The effects of organic residue quality on growth and reproduction of <Emphasis Type="Italic">Aporrectodea trapezoides</Emphasis> under different moisture conditions in a salt-affected agricultural soil

We studied the effects of organic residues with different C/N ratios and soil moisture contents on the growth and reproduction of the earthworm Aporrectodea trapezoides to investigate potential measures to increase its population in a salt-affected agricultural soil. The experiment consisted of eight treatments in a fully factorial design: low or high C/N ratio organic residue, soil moisture at 75 or 95% field capacity (FC), and salinity (as electrical conductivity (EC)) of 3.07 or 4.77 dS m^?1. It was carried out under controlled laboratory conditions for 4 months. In the low C/N ratio organic residue application, there was a significantly greater mean total dry weight and number of clitellate individuals of A. trapezoides, regardless of the soil moisture and salinity content, which may be due to the greater soil microbial biomass and dissolved organic N (DON) derived from the low C/N ratio organic residue. Generally, more cocoons were found in the application of low C/N ratio clover residue at months 2 and 4. At an EC of 3.07 dS m^?1 and moisture content of 75% field capacity (FC), significantly more hatchlings were found when low C/N ratio clover residue was applied compared to the high C/N ratio wheat residue. High soil moisture content (95% FC) resulted in a significantly greater mean total dry weight of A. trapezoides at months 2 and 4 and significantly more clitellate individuals and cocoons at month 4 compared to the low soil moisture content (75% FC), but only when the low C/N ratio residue was applied. In contrast, high soil moisture content (95% FC) resulted in significantly less hatchling numbers at an EC of 3.07 dS m^?1, only when the low C/N ratio residue was applied. These results suggest that the organic residue type and soil moisture content can regulate the growth and reproduction of the earthworm A. trapezoides, which should help to improve the recovery of their populations in salt-affected agricultural soil.     

A field study of gross rates of N mineralization and nitrification and their relationships to microbial biomass and enzyme activities in soils treated with dairy effluent and ammonium fertilizer

Abstract. Gross N mineralization and nitrification rates were measured in soils treated with dairy shed effluent (DSE) (i.e. effluent from the dairy milking shed, comprising dung, urine and water) or ammonium fertilizer (NH₄Cl) under field conditions, by injecting ¹⁵N-solution into intact soil cores. The relationships between gross mineralization rate, microbial biomass C and N and extracellular enzyme activities (protease, deaminase and urease) as affected by the application of DSE and NH₄Cl were also determined. During the first 16 days, gross mineralization rate in the DSE treated soil (4.3–6.1 μg N g^?1 soil day^?1) were significantly (P 14;< 14;0.05) higher than those in the NH₄Cl treated soil (2.6–3.4 μg N g^?1 soil day^?1). The higher mineralization rate was probably due to the presence of readily mineralizable organic substrates in the DSE, accompanied by stimulated microbial and extracellular enzyme activities. The stable organic N compounds in the DSE were slow to mineralize and contributed little to the mineral N pool during the period of the experiment. Nitrification rates during the first 16 days were higher in the NH₄Cl treated soil (1.7–1.2 μg N g^?1 soil day^?1) compared to the DSE treated soil (0.97–1.5 μg N g^?1 soil day^?1). Soil microbial biomass C and N and extracellular enzyme activities (protease, deaminase and urease) increased after the application of the DSE due to the organic substrates and nutrients applied, but declined with time, probably because of the exhaustion of the readily available substrates. The NH₄Cl application did not result in any significant increases in microbial biomass C, protease or urease activities due to the lack of carbonaceous materials in the ammonium fertilizer. However, it did increase microbial biomass N and deaminase activity. Significant positive correlations were found between gross N mineralization rate and soil microbial biomass, protease, deaminase and urease activities. Nitrification rate was significantly correlated to biomass N but not to the microbial biomass C or the enzyme activities. Stepwise regression analysis showed that the variations of gross N mineralization rate was best described by the microbial biomass C and N.     

Microbial Biomass in Relation to Soil Properties under Integrated Farming Systems of Meghalaya,India

The effects of different integrated farming systems on microbial biomass was studied 20 years after their adoption at Meghalaya, India. The soil fertility was relatively greater in agricultural and agri‐horti‐silvi‐pastoral systems as a result of accumulation of leaf litter/crop residues and addition of inorganic and organic manures. Microbial biomass carbon was greatest in agricultural (378 mg kg^?1) followed by the agri‐horti‐silvi‐pastoral systems (291 mg kg^?1). The most microbial biomass nitrogen (N) and phosphorus (P) (32.4 and 17.07 mg kg^?1, respectively) were recorded in agricultural followed by agri‐horti‐silvi‐pastoral systems. Microbial biomass carbon (C) had a significant relationship with organic C, microbial biomass N, and biomass P, indicating that the living part of soil organic matter is involved in the transformation of nutrients into the labile pool and governs their availability to the plants. Application of inorganic fertilizers and organics along with lime has contributed more microbial biomass that led to more biological activity attributed in nutrient transformations and also maintained the soil fertility.     

Ecological study on the dynamics of soil organic matter and its related properties in shifting cultivation systems of Northern Thailand

Abstract

There is a large number of hill people in northern Thailand, who practices shifting cultivation. In order to analyze the soil ecological problems involved in the transition from traditional shifting cultivation to more intensive upland farming, the authors carried out comparative studies on the dynamics of organic matter and its related properties in soils both in the traditional shifting cultivation systems adopted by Karen people and more intensive upland farming practiced by Thai and Hmong people in the area. The contents of organic matter and available N in the surface 10 cm layers of soil from the fields continuously cultivated were lower than those in soils under prolonged fallow (more than 10 y) or natural forest. Based on the rate of soil respiration, the amount of organic matter decomposed within 1 y was estimated to reach nearly 10% of that stored in the upper 50 cm layers of the soil profile in the upland crop fields. These results indicate that the organic matter-related resources markedly decreased under continuous cropping. The contents of C, N, and P in the microbial biomass of the surface 10 cm layers of soil ranged from 0.37 to 2.09 mg C g^?l soil, from 22.7 to 188 µg N g^?l soil, and from 6.1 to 65.7 µg P g^?l soil, respectively. Since the contents of microbial C, N, and P in the surface soils were generally higher under prolonged fallow and natural forests than in the fields continuously cultivated, the microbial activity and/or the amounts of C, N, and P available for biological activity seemed to have declined under continuous upland farming. The incubation experiment to assess the N mineralization pattern showed two remarkable characteristics: 1) there was an initial time lag until active mineralization of N occurred in the soils from young fallow forest and 2) the soil burning effect was observed after burning in the fields under prolonged fallow. The active process of nitrification after N mineralization was always associated with a sharp fall in soil pH, suggesting that soil acidification was promoted and basic cations were lost from the soils. In conclusion, rapid deterioration of the soil organic matter-related properties in cropping fields can be considered to be one of the ecological reasons why upland fields must be returned to fallow again a few years after forest reclamation in traditional shifting cultivation systems. Therefore, in alternative farming systems with more intensive land use, it is essential to apply organic materials into soils to decrease the rate of soil degradation, or to improve the soil fertility, in avoiding soil acidification along with nitrification.