The role of plant residues in pH change of acid soils differing in initial pH

Reports on the effect of plant residues on soil pH have been contradictory. The conflicting accounts have been suggested to result from differences in compositions and types of plant residues and characteristics of soils. This incubation study examined the effect of plant residues differing in concentrations of N (3-49 g kg⁻¹) and of alkalinity (excess cations) (220-1560 mmol kg⁻¹) on pH change of three soils differing in initial pH (3.9-5.1 in 0.01 M CaCl₂). The addition of plant residues at a rate of 15 g kg⁻¹ soil weight increased the pH of all soils by up to 3.4 units and the pH reached the maximum at day 42 after incubation for Wodjil (initial pH 3.87) and Bodallin (pH 4.54) soils and day 14 for Lancelin soil (pH 5.1). The amount of pH buffering was decreased by residue addition in Wodjil soil, increased in Bodallin soil and remained unchanged in Lancelin soil, which closely related to changes of soil pH. Residue addition increased concentration and the increase in concentration generally correlated positively with the concentration of residue N. The concentration increased with time, reached the peak at Days 42-105 for Wodjil soil, Days 14-105 for Bodallin soil and Days 14-42 for Lancelin soil, and then decreased only in Lancelin soil. The concentration of was kept minimal in Wodjil and Bodallin soils. In Lancelin soil, concentrations increased with incubation time from days 14-28. Irrespective of plant residue and incubation time, the amounts of alkalinity produced due to residue addition correlated highly with the sum of the alkalinity added as plant residues (excess cations) and those resulting from mineralization of residue N, with the slope of regression lines decreasing with increase of the initial soil pH. Direct shaking of soil with the residues at the same rate of alkalinity (excess cations) under sterile conditions increased the pH of the Wodjil soil but decreased it in the Lancelin soil. It is suggested that the decarboxylation of organic anions (as indicated by excess cations) of added plant residues and ammonification of the residue N causes soil pH increase whereas nitrification of mineralised residue nitrogen causes soil pH decrease, and that the association/dissociation of organic compounds also plays a role in soil pH change, depending initial pH of the soil. The overall effect on soil pH after addition of plant residues would therefore depend on the extent of each of these processes under given conditions.     

添加黏土及植物残体混合物对沙质土壤中呼吸作用和微生物量碳、氮有效性的影响

Crop yields in sandy soils can be increased by addition of clay-rich soil, but little is known about the effect of clay addition on nutrient availability after addition of plant residues with different C/N ratios. A loamy sandy soil(7% clay) was amended with a clay-rich subsoil(73% clay) at low to high rates to achieve soil mixtures of 12%, 22%, and 30% clay, as compared to a control(sandy soil alone) with no clay addition. The sandy-clay soil mixtures were amended with finely ground plant residues at 10 g kg~(-1): mature wheat(Triticum aestivum L.) straw with a C/N ratio of 68, mature faba bean(Vicia faba L.) straw with a C/N ratio of 39, or their mixtures with different proportions(0%–100%, weight percentage) of each straw. Soil respiration was measured over days 0–45 and microbial biomass C(MBC), available N, and p H on days 0, 15, 30, and 45. Cumulative respiration was not clearly related to the C/N ratio of the residues or their mixtures, but C use efficiency(cumulative respiration per unit of MBC on day 15) was greater with faba bean than with wheat and the differences among the residue mixtures were smaller at the highest clay addition rate. The MBC concentration was lowest in sole wheat and higher in residue mixtures with 50% of wheat and faba bean in the mixture or more faba bean. Soil N availability and soil p H were lower for the soil mixtures of 22% and 30% clay compared to the sandy soil alone. It could be concluded that soil cumulative respiration and MBC concentration were mainly influenced by residue addition, whereas available N and p H were influenced by clay addition to the sandy soil studied.     

Impact of organic matter addition on pH change of paddy soils

Purpose

The objective of the present study was to explore the effect of initial pH on the decomposition rate of plant residues and the effect of residue type on soil pH change in three different paddy soils.

Materials and methods

Two variable charge paddy soils (Psammaquent soil and Plinthudult soil) and one constant charge paddy soil (Paleudalfs soil) were used to be incubated at 45 % of field capacity for 105 days at 25 °C in the dark after three plant residues (Chinese milk vetch, wheat straw, and rice straw) were separately added at a level of 12 g?kg^?1 soil. Soil pH, CO₂ escaped, DOC, DON, MBC, MBN, NH ₄ ⁺ , and NO ₃ ^? during the incubation period were dynamically determined.

Results and discussion

Addition of the residues increased soil pH by 0.1–0.8 U, and pH reached a maximum in the Psammaquent and Plinthudult soils with low initial pH at day 105 but at day 3 in the Paleudalfs soil with high initial pH. Incorporation of Chinese milk vetch which had higher concentration of alkalinity (excess cations) and nitrogen increased soil pH more as compared with incorporation of rice and wheat straws. Microbial activity was the highest in Chinese milk vetch treatment, which resulted in the highest increase of soil pH as compared with addition of rice and wheat straws. However, nitrification seemed to be inhibited in the variable charge soils of Psammaquent and Plinthudult but not in the constant charge soil of Paleudalfs.

Conclusions

The effectiveness of increasing soil pH after incorporation of the plant materials would be longer in low initial pH soils of Psammaquent and Plinthudult than in high initial pH soil of Paleudalfs. In order to achieve the same degree of pH improvement, higher amounts of plant residues should be applied in constant charge soils than in variable charge soils.     

Addition of a clay subsoil to a sandy topsoil changes the response of microbial activity to drying and rewetting after residue addition – a model experiment

The effect of drying and rewetting (DRW) on C mineralization has been studied extensively but mostly in absence of freshly added residues. But in agricultural soils large amounts of residues can be present after harvest; therefore, the impact of DRW in soil after residue addition is of interest. Further, sandy soils may be ameliorated by adding clay‐rich subsoil which could change the response of microbes to DRW. The aim of this study was to investigate the effect of DRW on microbial activity and growth in soils that were modified by mixing clay subsoil into sandy top soil and wheat residues were added. We conducted an incubation experiment by mixing finely ground wheat residue (20 g kg^–1) into top loamy sand soil with clay‐rich subsoil at 0, 5, 10, 20, 30, and 40% (w/w). At each clay addition rate, two moisture treatments were imposed: constantly moist control (CM) at 75% WHC or dry and rewet. Soil respiration was measured continuously, and microbial biomass C (MBC) was determined on day 5 (before drying), when the soil was dried, after 5 d dry, and 5 d after rewetting. In the constantly moist treatment, increasing addition rate of clay subsoil decreased cumulative respiration per g soil, but had no effect on cumulative respiration per g total organic C (TOC), indicating that the lower respiration with clay subsoil was due to the low TOC content of the sand‐clay mixes. Clay subsoil addition did not affect the MBC concentration per g TOC but reduced the concentration of K₂SO₄ extractable C per g TOC. In the DRW treatment, cumulative respiration per g TOC during the dry phase increased with increasing clay subsoil addition rate. Rewetting of dry soil caused a flush of respiration in all soils but cumulative respiration at the end of the experiment remained lower than in the constantly moist soils. Respiration rates after rewetting were higher than at the corresponding days in constantly moist soils only at clay subsoil addition rates of 20 to 40%. We conclude that in presence of residues, addition of clay subsoil to a sandy top soil improves microbial activity during the dry phase and upon rewetting but has little effect on microbial biomass.     

Soil respiration,glomalin content,and enzymatic activity response to straw application in a wheat-maize rotation system

Purpose

Straw residue has been widely applied in the North China Plain agroecosystems due to their positive roles in soil fertility improvement, sustainable production, and climate change mitigation. However, little is known about how straw application alters soil respiration by influencing soil biochemical properties in this region. This is the first study to evaluate the role of soil enzyme activity and glomalin content in the response of soil respiration to straw application at different growth stages in a wheat-maize rotation system.

Materials and methods

Field experiment was conducted in a wheat-maize rotation system and it contained two treatments: straw residue removal (CK) and straw residues application (SR). Soil respiration, moisture, and temperature were measured using LI-8100 at different growth stages during wheat and maize (2013–2015) growing seasons. From 2013 to 2014, soil sample (0–20 cm) was collected at different growth stages during wheat and maize growing seasons and transported to the laboratory. Glomalin content and soil enzyme activity were analyzed by using Bradford and enzyme-labeled meter method, respectively. In addition, we determined soil chemical properties such as soil organic carbon (SOC), soil total N content (TN), ammonium N (NH₄ ⁺-N), and nitrate N (NO₃ ^?-N) concentrations.

Results and discussion

SR significantly increased soil respiration and this promotion effect became more significant after 4-year straw application. Soil respiration exhibited significant seasonal variation and was significantly increased by soil temperature with Q ₁₀ ranging from 1.73 to 2.14 for CK and from 1.51 to 2.28 for SR. Both soil temperature and moisture accounted for 70–72% of the seasonal variation in soil respiration. SR significantly increased easily extractable glomalin-related soil protein during 2013–2014 wheat growing season except jointing stage. In addition, positive and significant effect of SR on activities of β-glucosidase and cellobiohydrolase was observed at initial and vigorous growth stages. Straw application significantly increased TN, but did not significantly influence SOC, NH₄ ⁺-N, and NO₃ ^?-N concentrations.

Conclusions

Our study demonstrated that straw application increased soil respiration by stimulating soil enzyme activities and improving easily extractable glomalin-related soil protein. Straw application is recommended as an agricultural management in the North China Plain because of its role in improving biochemical properties. To improve soil biochemical parameters with a relative low soil respiration rate, further information is necessary about the optimum amount of straw application.

     

Application of mixed straw and biochar meets plant demand of carbon dioxide and increases soil carbon storage in sunken solar greenhouse vegetable production

Solar vegetable greenhouse soils show low soil organic carbon content and thus also low rates of soil respiration. Processing vegetable residues to biochar and mixing biochar with maize straw might improve soil respiration and increase soil organic carbon stocks, while preventing the spread of soil-borne diseases carried by vegetable residues. In an incubation experiment, we tested how additions of maize straw (S) and biochar (B) added in varying ratios (100S, 75S25B, 50S50B, 25S75B, 100B and 0S0B (control)) affect soil respiration and fraction of added C remaining in soil. Daily CO₂ emissions were measured over 60 days incubation, the natural abundance of ¹³C in soil and in the added biochar and maize straw were analysed. Our result shows that (a) soil CO₂ emissions were significantly increased compared to soil without the straw additions, while addition of biochar only decreased soil respiration; (b) cumulative CO₂ emissions decreased with increasing ratio of added biochar to maize straw; (c) the abundance of soil ¹³C was significant positively correlated with cumulative CO₂ emissions, and thus with the ratio of straw addition. Our results indicate that incorporation of maize straw in greenhouse soils is a meaningful measure to increase soil respiration and to facilitate greenhouse atmosphere CO₂ limitation while producing vegetables. On the other hand, additions of biochar from vegetable residues will increase soil organic carbon concentration. Therefore, the simultaneous application of maize straw and biochar obtained from vegetable residues is an effective option to maintain essential soil functions for vegetable production in sunken solar greenhouses.     

Effect of mixing soil saprophytic fungi with organic residues on the response of Solanum lycopersicum to arbuscular mycorrhizal fungi

The effect of the dual inoculation with arbuscular mycorrhizal (AM) and saprophytic fungi and a combination of wheat straw and sewage sludge residues were studied by determining their effect on dry weight of tomato and on chemical and biochemical properties of soil. Incubation of organic residue (sewage sludge combined with wheat straw) with saprophytic fungi and plant inoculation with mycorrhizal fungi was essential to study plant growth promotion. Soil application of organic residues increased the dry weight of tomato inoculated with Rhizophagus irregularis. The greatest shoot dry mass was obtained when the organic residues were incubated with Trichoderma harzianum and applied to AM plants. However, the greatest percentage of root length colonized with AM in the presence of the organic residues was obtained with inoculation with Coriolopsis rigida. The relative chlorophyll was greatest in mycorrhizal plants regardless of the presence of either saprophytic fungus. The presence of the saprophytic fungi increased soil pH as the incubation time increased. Soil nitrogen and phosphorus contents and acid phosphatase were stimulated by the addition of organic residues, and contents of N and P. Total N and P content in soil increased when the organic residue was incubated with saprobe fungi, but this effect decreased as the incubation period of the residue with saprobe fungi increased. The same trend was observed for soil β‐glucosidase and fluorescein diacetate activities. The application of organic residues in the presence of AM and saprophytic fungi seems to be an interesting option as a biofertilizer to improve plant growth and biochemical parameters of soils.     

Carbon dioxide evolution from wheat and lentil residues as affected by grinding,added nitrogen,and the absence of soil

Summary A study was conducted to determine the effects of grinding, added N, and the absence of soil on C mineralization from agricultural plant residues with a high C:N ratio. The evolution of CO₂ from ground and unground wheat straw, lentil straw, and lentil green manure, with C:N ratios of 80, 36, and 9, respectively, was determined over a period of 98 days. Treatments with added N were included with the wheat and lentil straw. Although the CO₂ evolution was initially much faster from the lentil green manure than from the lentil or wheat straw, by 98 days similar amounts of CO₂ had evolved from all residues incubated in soil with no added N. Incubation of plant residues in the absence of soil had little effect on CO₂ evolution from the lentil green manure or lentil straw but strongly reduced CO₂ evolution from the wheat straw. Grinding did not affect CO₂ evolution from the lentil green manure but increased CO₂ evolution from the lentil straw with no added N and from the wheat straw. The addition of N increased the rate of CO₂ evolution from ground wheat straw between days 4 and 14 but not from unground wheat straw, and only slightly increased the rate of CO₂ evolution from lentil straw during the initial decomposition. Over 98 days, the added N reduced the amounts of CO₂ evolved from both lentil and wheat straw, due to reduced rates of CO₂ evolution after ca. 17 days. The lack of an N response during the early stages of decomposition may be attributed to the low C:N ratio of the soluble straw component and to microbial adaptations to an N deficiency, while the inhibitory effect of N on CO₂ evolution during the later stages of decomposition may be attributed to effects of high mineral N concentrations on lignocellulolytic microorganisms and enzymes.     

Effects of plant residues and environmental factors on phosphorus availability in soils

Abstract

Phosphorus availability is a major nutritional problem in several northern Idaho soils. Traditionally, fertilizers containing P have been applied to improve availability in soils; however, organic materials added to soils have the ability to provide large quantities of labile P via mineralization processes and to reduce sorption of P. Using this concept, plant residues applied to soils would increase P availability for future plant needs. This research evaluated the effect of plant residue, incorporated into a Northern Idaho soil, on P availability under controlled laboratory conditions. Alfalfa (Medicago sativa), pea (Pisum sativum) and wheat (Triticum aestivum) plant residues were incorporated into soil collected from the Ap horizon of a Latahco silt loam (fine‐silty, mixed, frigid Argiaquic Xeric Argialboll) at rates of 0, 1, 5 and 10% (w/w). The soils were incubated at soil water potentials of ‐0.05, ‐0.15 and ‐0.40 MPa, and temperatures of 10, 20 and 30°C over a 20 week period. Soils were sampled at 2, 4, 8, 12, 16 and 20 weeks for determination of NaOAc extractable P. Data were analyzed by SAS‐GLM and Omega squared (ω²) values were used to identify the impact of each main effect and interaction. A significant 4‐factor interaction of plant residue x amendment rate x water potential x incubation time, four 3‐factor interactions, six 2‐factor interactions and four main effects were observed at each of the three incubation temperatures. Since all interactions and main effects significantly affected P availability, ω² values were used to assess their relative importance. Amendment rate was the most important factor and plant residue material was the second most important factor observed affecting extractable P levels. In general, NaOAc extractable P increased with increasing amendment rates and incubation time‐period. Increasing incubation temperature and soil water potential also positively affected the extracted P level. The greatest amount of P was mineralized from alfalfa residue material while the smallest amount was released from wheat residue. Pea residue contributed an intermediate quantity of extractable P. This study demonstrated that residues applied to northern Idaho soils have the ability to enhance P availability in addition to providing a usable N source.     

The effect of plant material on the relationship between sodium acetate and sodium bicarbonate extractable phosphorus in a Latahco silt loam soil

Abstract

Standardization of the P soil test procedures is desirable; however, both NaOAc and NaHCO₃ are currently used to extract P from soils in the Pacific Northwest region of the USA. The purpose of this study was to determine the relationship between NaOAc and NaHCO₃ extractable P in soils and to evaluate the effect of plant material on this relationship in a northern Idaho soil. The Ap horizon of a Latahco silt loam was used and alfalfa (Medicago sativa), pea (Pisum sativum) and wheat (Triticum aestivum) plant materials were added as amendments at rates of 0%, 1%, 5% and 10% (w/w). The soils were incubated for 20 weeks under controlled conditions. In addition, other parameters studied included soil water potential (‐0.05, ‐0.15 and ‐0.40 MPa), incubation temperature (10, 20 and 30°C and incubation period. P in samples was extracted by NaOAc and NaHCO₃ extractants. A statistically significant linear relationship between NaOAc and NaHCO₃ extractable P was observed (r² = 0.96). In addition, the types of plant residues added to soil differently affected P extraction by the two extractants. The difference between NaOAc and NaHCO₃ extractable P was greatest in the wheat material treatment while alfalfa material resulted in the smallest effect. Sodium acetate extractable P values increased faster than NaHCO₃ extractable P with increasing amendment rate.

A simple regression relationship will allow conversion between NaOAc and NaHCO₃ extractable P in the Latahco soil. Additions of less than 5 mt/ha plant material will have a minimal impact on this relationship.     

The influence of plant residues on soil aggregation and carbon content: A meta-analysis

Background

Soil aggregation and organic carbon (OC) content are important indicators of soil quality that can be improved with plant residue amendments. The extent of the effects of plant residue amendments on soil aggregation and OC content across different plant residue and soil types is not fully understood.

Aim

In this meta-analysis, we evaluated the effects of plant residue amendments on soil aggregation and OC content for different plant residues (fresh, charred) and soil types varying in clay content, initial OC content, and pH.

Methods

Our meta-analysis included 50 published studies (total of 299 paired observations). We estimated the response ratios of mean weight diameter (MWD) and separate aggregate size classes, total soil OC (TSC), and aggregate-associated OC. We also considered the effect of experimental factors (study duration, residue type, residue amount, initial soil OC, clay content, and pH).

Results

The benefit of plant residue amendment on soil aggregation was larger in soils with initially low OC content and neutral pH. Initial soil OC content and pH were more important than soil clay content for OC storage in soil aggregates. Both fresh and charred plant residue amendments were effective in forming aggregates, whereas charred residues were more effective in increasing TSC. We found only a weak positive relationship between the response ratio of TSC and MWD indicating that other factors besides soil aggregation contributed to the increase in soil C storage.

Conclusions

While plant residue amendments can enhance soil aggregation and TSC, these effects are likely governed by the type of plant residue and soil properties such as the initial soil pH and OC content.     

Mineralization of sulfur from organic residues assessed by inverse isotope dilution

Sulfur (S) deficiency in soils is increasingly recognized in agricultural systems. The quantification of S mineralization/immobilization processes after incorporation of organic materials into soils is a key factor to predict the availability of S to growing plants. However, immobilization and mineralization occur simultaneously making the quantification of the magnitude of each process difficult. We used the inverse isotope (³⁵SO₄) dilution technique to quantify immobilization and mineralization fluxes after incorporation of two organic residues with contrasting C/S ratio's (cabbage or wheat straw) into a sandy soil in planted and unplanted soils (pot trial with ryegrass and incubation). The soil was labeled with ³⁵SO₄ and incubated for 63 days prior to the application of residues. The specific activity (SA) of soil-extractable SO₄ did not change significantly in the control soil during the subsequent experimental period despite significant net mineralization, illustrating that labile-S in soil was homogenously labeled. Application of residues decreased the SAs during the incubation due to the dilution with unlabeled-S from the residues. A three-compartment dynamic model was fitted to the SA data predicting that gross mineralization of residue-S was almost complete over 43 days incubation although this release was not matched by the increase in soil SO₄ due to immobilization reactions. Soil-extractable SO₄ was significantly increased in the cabbage-treated soil while the reverse was true in the wheat straw amended soil in which the S-immobilization was almost twice the gross mineralization of residue-S. The SA of S in ryegrass were maximally 15% lower than in corresponding soil extracts suggesting that residue mineralization was similar in planted and unplanted soils. The inverse isotope dilution method offers potential for screening S release of different residues; however the details of the dynamics of soil-S isotopes show that the individual fluxes are not constant during the incubation.     

Enhancing Soil Quality through Residue Management in a Rice-Wheat System in Fukuoka,Japan

A long-term experiment (LTE) on a rice-wheat system was initiated in 1963 at the Kyushu National Agricultural Experiment Station, in Fukuoka, Japan, to determine the effects of continuous application of rye grass/wheat straw, rice straw and rice straw compost, alone or in combination with inorganic N on crop yields. Increase in rice yields and enhancement of total soil C and N contents with the application of organic residues in this LTE have been reported earlier. However, evaluation of the changes in the soil microbiological properties and the decomposable C fraction of soil organic matter that is needed for soil quality assessment is still lacking. Soil samples were collected after rice harvest in 2003 from the organic residue treatments and unfertilized control, air-dried and incubated for 1 month under aerobic [50% water-filled pore space (WFPS)] and flooded conditions prior to the analysis of the amount of microbial biomass C (MBC), soil respiration and the amount of potential mineralizable N (PMN). The contents of total C (TC), total N (TN), organic C (OC), hot water-extractable C (HWEC) and permanganate-oxidizable C (POC) were determined from air-dried soils. Organic residue incorporation brought about significant increases in the contents of TC, TN, OC, POC, HWEC and PMN. The largest accumulation of total C (23%) and N (72%) in the soil was from rice straw compost, compared with that from rice straw (C, 7% and N, 33%) and rye grass/wheat straw (C, 9% and N, 29%). Incorporation of rice straw compost also increased the amount of MBC under both aerobic and flooded conditions and basal soil respiration under aerobic conditions only. An efficient utilization of C by microorganisms was indicated by a significantly lower metabolic quotient (qCO₂) in the composted and uncomposted rice straw treatments compared with the control in the “-” N treatment under aerobic conditions. Similarly, the flush of CO₂ after rewetting of dry soil per unit of HWEC was lower in the organic matter treatments, indicating a more efficient C utilization and lower C losses per unit of available C. The content of HWEC was significantly correlated with the basal soil respiration (at 50% WFPS), the amounts of MBC, PMN and with the increase in the content of soil organic C in the residuetreated soils. In the treatments without inorganic N fertilizer, grain yield was significantly correlated with the amounts of total organic C, HWEC, MBC (at 50% WFPS), basal soil respiration (at 50% WFPS) and the amount of PMN.     

Effect of plant residue return on the development of surface soil pH gradients

 In the field, surface soil pH gradients were observed under senescing plants over late spring and summer. A soil incubation experiment was conducted (119 days, 20  °C) to provide direct evidence of the influence of plant residue incorporation on soil pH. This was investigated in terms of plant residue type (wheat and subterranean clover) and dry matter addition rate (0, 6.25, 12.5 and 25.0 g kg^–1), as well as the soil layer of incorporation (0–2.5 and 7.5–10 cm) and moisture regime (continuously moist and moist-dry cycles). During incubation, moist unamended soils slowly acidified. In contrast, the addition of plant residue resulted in a rapid (day 0–7) increase of soil pH due to the association, and particularly oxidation, of added organic anions. This was followed by a gradual (day 7–119) pH decline attributed to the mineralization and subsequent nitrification of added organic N. The addition of 12.5–25.0 g kg^–1 of cereal crop residues, and 6.25–25.0 g kg^–1 of legume-based pasture residues, resulted in a net alkalization of the surface 2.5 cm of soil. It was therefore concluded that surface soil pH gradients observed in the field were largely attributable to an increase of pH at the surface 2.5 cm in response to plant residue return. The magnitude of such gradients will be particularly large with the return of large quantities of plant residues of high ash alkalinity in soils of relatively low initial pH and biological activity, and when the surface of the soil is exposed to moist-dry cycles. Received: 11 October 1999     

Community composition and activity of microbes from saline soils and non-saline soils respond similarly to changes in salinity

Saline soils are wide-spread and characterised by poor plant growth and low microbial activity but salinity fluctuates seasonally or with irrigation water quality. Therefore it is important to understand the response of soil microbial communities to changes in soil salinity. We carried out an experiment to test the hypothesis that microbial communities from soils with medium to high salinity respond differently to salinity than microbes from non-saline soils or soils with low salinity. We prepared a microbial inoculum from field soils of different salinity (EC_1:5 0.3, 1.1, 2.7, 4.6 and 6.0 dS m⁻¹). This inoculum was added to quartz sand adjusted to EC_1:5 0.3, 1.1, 2.9, 4.6, 6.0 and 8.0 dS m⁻¹ and amended with finely ground wheat straw and basal nutrients. The sand mix was incubated at 80% water holding capacity for 27 days. Soil respiration was measured continuously, microbial community composition (based on phospholipid fatty acid analysis) and particulate organic carbon (POC) were determined at the start and the end of the incubation. Irrespective of inoculum EC, cumulative respiration decreased with increasing adjusted EC with no differences among inocula. The POC concentration was always lowest at adjusted EC 0.3 and highest at EC 8.0. Up to adjusted EC 4.6, the POC concentration was lower with inoculum EC 0.3 than with the inocula of higher EC. The inocula had distinct microbial community composition at all adjusted ECs, but the changes induced by the adjusted EC were similar in all inocula. The results are contrast to our hypothesis because increasing salinity decreased soil respiration of all inocula to a similar extent. In fact, the lower POC concentration with inoculum from the non-saline soil up to an adjusted EC of 4.6 suggests that the microbial communities from the non-saline soil are able to decompose the added wheat straw under low to moderate salinity to a greater extent than those from saline soils. On the other hand, even microbes from highly saline soils can respond quickly with an increase in activity if the salinity is reduced, e.g. after heavy rainfall which leaches the salts out of the top soil.     

Temporal variations of crop residue effects on soil N transformation depend on soil properties as well as residue qualities

Purple soils (Eutric Regosols) are widely distributed in humid subtropical Southwest China. They are characterized by high nitrification activities, with risks of severe NO₃^? leaching. Incorporation of crop residues is considered an effective method to reduce NO₃^? loss. In the present study, we compared the effects of alfalfa, rice straw, and sugarcane bagasse on gross N transformation turnover in a purple soil (purple soil, pH 7.62) compared with those in an acid soil (acid soil, pH 5.26), at 12 h, 3 months, and 6 months after residue incorporation. The gross N transformation rates were determined by ¹⁵N tracing. All tested crop residues stimulated the gross N mineralization rates, but reduced the net mineralization rates in both soils at 12 h after residue incorporation; however, the extent of the effect varied with the crop residue qualities, with rice straw having the strongest effects. Crop residues reduced net nitrification rates by depressing gross autotrophic nitrification rates and stimulating NO₃^? immobilization rates in the purple soil, particularly after rice straw incorporation (net nitrification rate decreased from 16.72 mg N kg^?1 d^?1 in the control to ??29.42 mg N kg^?1 d^?1 at 12 h of residue incorporation); however, crop residues did not affect the gross autotrophic nitrification rates in the acid soil. Crop residue effects subsided almost completely within 6 months, with sugarcane bagasse showing the longest lasting effects. The results indicated that crop residues affected the N transformation rates in a temporal manner, dependent on soil properties and residue qualities.     

RELATIONSHIP OF SOIL EXTRACTABLE AND FERTILIZER BORON TO SOME SOIL PROPERTIES,CROP YIELDS,AND TOTAL BORON IN COTTON AND WHEAT PLANTS ON SELECTED SOILS OF PUNJAB,PAKISTAN

Boron (B) is an essential microelement, which is necessary for reproductive organs including pollen tube formation in wheat (Triticum aestivum L.), and flowering and boll formation in cotton (Gossypium hirsutum L.) The study was associated with wheat-cotton rotation in 80 farm fields, belonging to different soil series, in four districts of cotton belt of Punjab, Pakistan to assess concentrations of extractable B in soils [0.05 M hydrochloric acid (HCl) extractable B], and added fertilizer B and their relationship to some soil physico-chemical properties [pH, organic matter (OM), calcium carbonate (CaCO₃) and clay content], yields and total B concentrations in wheat and cotton plants. All soils had alkaline pH (7.45 to 8.55), high CaCO₃ content (2.14 to 8.65%), less than 1.0% OM (0.33 to 0.99%), low plant available-P (Olsen P less than 8 mg kg^?1 soil) and medium ammonium acetate extractable potassium (K) (< 200 mg K kg^?1 soil). Of the 80 soil samples, 65 samples (81%) were low in available B (<0.45 mg B kg^?1, ranging from 0.11 to 0.43 mg B kg^?1) Of the corresponding 80 plant samples, leaves B concentrations were below critical levels (<10 mg B kg^?1 for wheat; <30 mg B kg^?1 for cotton) for all the tested samples for wheat and cotton. The regression analysis between plant total B concentrations and soil extractable B concentrations showed strong linear positive relationships for both wheat (R² = 0.509***, significant at P <0.001) and cotton (R² = 0.525***, significant at P <0.001). Further regression analysis between extractable soil B and wheat grain yield as well as between wheat leaves total B and wheat grain yield also depicted strong linear relationships (R² = 0.76 and 0.42, respectively). Boron fertilizer demonstration plots laid out at farmers’ fields low in extractable B, in each district not only enhanced grain yields of wheat crop but also contributed a significant increase towards seed cotton yield of succeeding cotton crop through residual B effect. In conclusion, the findings suggest that many soils in the cotton belt of Punjab may be low in extractable B for wheat and cotton, especially when these crops are grown on low OM soils with high CaCO₃ content.     

Frequent addition of wheat straw residues to soil enhances carbon mineralization rate

In many ecosystems, residues are added frequently to soil, in the form of root turnover and litter fall. However, in most studies on residue decomposition, residues are added once and there are few studies that have investigated the effect of frequent residue addition on C mineralization and N dynamics. To close this knowledge gap, we mixed mature wheat residue (C/N 122) into soil at a total rate of 2% w/w once at the start (R1×), every 16 days (R4×), every 8 days (R8×) or every 4 days (R16×). Un-amended soil served as control. All treatments were mixed every 4 days. Soil respiration was measured continuously over the 80-day incubation. Inorganic N, K₂SO₄-extractable C and N, chloroform-labile C and N (as an estimate of microbial biomass C and N), soil pH and microbial community composition were assessed every 16 days. Increasing frequency of residue addition increased C mineralization per g residue. Compared to R1×, cumulative respiration per g residue at the end of the incubation (day 80) was increased by 57, 82 and 92% in R4×, R8× and R16×, respectively. The largest differences in soil respiration per g residue occurred in the first 30 days. Despite large increases in cumulative respiration, frequent residue addition did not affect inorganic N or K₂SO₄-extractable N concentrations, chloroform-labile C and N or soil pH. Compared to the control, all residue treatments resulted in increases in chloroform-labile C and N and soil pH but decreased inorganic and K₂SO₄-extractable N. Microbial community composition was affected by residue addition, however there were no consistent differences among residue treatments. It is concluded that experiments with single residue additions may underestimate residue decomposition rates in the field. The increased C mineralization caused by frequent residue additions does not appear to be due to an increased microbial biomass or changes in microbial community composition, but rather to increased C mineralization per unit biomass.     

Grain legume pre-crops and their residues affect the growth, P uptake and size of P pools in the rhizosphere of the following wheat

Legumes have been shown to increase P uptake of the following cereal, but the underlying mechanisms are unclear. The aim of this study was to compare the effect of legume pre-crops and their residues on the growth, P uptake and size of soil P pools in the rhizosphere of the following wheat. Three grain legumes (faba bean, chickpea and white lupin) were grown until maturity in loamy sand soil with low P availability to which 80?mg P kg^?1 was supplied. This pre-crop soil was then amended with legume residues or left un-amended and planted with wheat. The growth, P uptake and concentrations of P pools in the rhizosphere of the following wheat were measured 6?weeks after sowing. In a separate experiment, residue decomposition was measured over 42?days by determining soil CO₂ release as well as available N and P. Decomposition rates were highest for chickpea residues and lowest for wheat residues. P release was greatest from white lupin residues and N release was greatest from faba bean residues, while wheat residues resulted in net N and P immobilisation. The growth of the following wheat was greater in legume pre-crop soil without residue than in soils with residue addition, while the reverse was true for plant P concentration. Among the legumes, faba bean had the strongest effect on growth, P uptake and concentrations of the rhizosphere P pools of the following wheat. Regardless of the pre-crop and residue treatment, wheat depleted the less labile pools residual P as well as NaOH-Pi and Po, with a stronger depletion of the organic pool. We conclude that although P in the added residues may become available during decomposition, the presence of the residues in the soil had a negative effect on the growth of the following wheat. Further, pre-crops or their residues had little effect on the size of P pools in the rhizosphere of wheat.     

Contrasting Effects of Four Plant Residues on Phosphorus Sorption-Desorption in Some Phosphorus Fertilized Calcareous Soils

Four types of plant residues (fruit waste, potato, sunflower, and wheat) with wide ranges of carbon to nitrogen (C/N) and carbon to phosphorus (C/P) ratios were added to the soil at the rate of 20 g kg^?1 (dry weight basis) and incubated for two months. In soils treated with plant residues, the P sorption ranged from 62.0% (potato) to 96.6% (wheat) and from 12.6% (fruit waste) to 50.6% (wheat) when 20 and 1500 mg P kg^?1 were added to the soils, respectively. In general, incorporation of plant residues decreased maximum P sorption capacity but increased bonding energy. The maximum P sorption capacity was reduced from 586 mg kg^?1to 500, 542, and 548 by wheat, fruit, and potato residues, respectively, but increased to 665 mg kg^?1 by sunflower residue. At higher P addition, the highest percentage of desorbed P was observed in soils treated with wheat residue (49.9%); followed by fruit waste (46.5%), potato (43.5%), sunflower (38.8%) and control soils (37.0%). It indicated that the P content of the organic residues had an important role in the sorption and desorption of P in calcareous soils. Among organic residues, sunflower residue showed high sorption and low desorption of P in soils, indicating a higher potential of this organic residue for P retention and reducing surface and groundwater contamination in calcareous soils.