Contribution of soluble and insoluble fractions of agricultural residues to short‐term pH changes

The retention of agricultural residues in cropping systems to maintain soil fertility is also important for the redistribution of alkalinity. In systems that adopt minimum or no‐tillage practices residue incorporation into the soil may occur slowly and the contribution of soluble and insoluble residue fractions to pH change may vary temporally and spatially. In this study we examined the contribution of whole, water soluble (70°C for 1 hour for two cycles) and insoluble fractions of canola, chickpea and wheat residues (added at 10 g kg^?1 soil) to pH change in a Podosol (Podzol; initial pH 4.5) and a Tenosol (Cambisol; initial pH 6.2) over a 59‐day incubation period. Whole residues increased pH in both soils, with the magnitude of the pH increase (chickpea > canola > wheat) being related to alkalinity content (concentration of excess cations) of the residue. Temporal release of alkalinity was only observed for the larger alkalinity content canola and chickpea residues and the change in pH was greater than during the initial period (approximately 4 hours; T0). Increases in pH were attributed to the decarboxylation of organic anions and the association of H⁺ with organic anions and other negatively charged chemical functional groups. The relative contribution of these processes depended on the residue and the initial soil pH. Our results show that 40–62% of the alkalinity of canola and chickpea residues resided in the soluble fraction. Furthermore, pH increases caused by soluble fractions may be transient if these contain large N concentrations. Soil properties that influence inorganic N dynamics such as inhibition of nitrification at acid pH will be important in determining the subsequent direction and magnitude of pH change.     

通过调节土壤氮素转化提高有机物料对红壤酸度的改良效果

通过培养试验,比较研究了油菜秸秆、稻草、香樟叶和豌豆秸秆单独施用以及油菜秸秆、稻草和香樟叶与豌豆秸秆混合施用对红壤酸度的改良效果。结果表明,在60天培养期内,添加4种物料均提高了土壤pH。培养试验结束时香樟叶、油菜秸秆、豌豆秸秆和稻草分别使土壤pH相对对照增加0.53、0.42、0.30和0.26。对于灰化碱含量很高的非豆科物料如香樟,其对土壤酸度的改良效果主要来源于物料所含碱性物质和物料对土壤硝化反应的抑制,但对灰化碱含量较低的非豆科物料如油菜秸秆和稻草,其改良效果主要来源于后者。豆科类豌豆秸秆主要通过所含碱性物质和有机氮矿化提高土壤pH,但培养试验后期铵态氮硝化反应释放的质子抵消了其部分改良效果。将油菜秸秆、稻草和香樟叶与豌豆秸秆配合施用,使硝化反应受到一定程度的抑制,提高了物料对土壤酸度的改良效果。培养试验结束时,香樟叶、稻草和油菜秸秆与豌豆秸秆配合施用比豌豆秸秆单独施用土壤pH分别高0.25、0.18和0.12。研究发现,香樟叶灰化碱含量很高,无论单独施用,还是与豌豆秸秆配合施用均有很好的改良效果,因此在南方地区推广种植香樟可以通过其凋落物修复酸化的森林土壤。     

湿地松林土壤生化特性和酶活性对模拟硫沉降的响应

以亚热带湿地松人工林为研究对象,通过3种水平(对照CK:pH 6.5;低硫LS:pH 4.5;高硫HS:pH 2.5)的模拟硫沉降控制试验,分析土壤生化特性及酶活性对硫添加的响应。结果表明:(1)硫输入促进了土壤酸化,0—5 cm土层土壤pH在HS处理下显著降低,5—10 cm土层土壤pH在LS和HS处理下显著降低(P0.05);(2)硫输入对土壤有机碳库存在一定影响,土壤总有机碳(TOC)对硫输入无显著响应,但土层间的差异性显著增加(P0.05),土壤可溶性有机碳(DOC)受影响有限,5—10 cm土层微生物量碳(MBC)LS显著降低(P0.05);(3)硫输入对土壤有效氮库影响存在差异,土壤可溶性有机氮(DON)、铵态氮(NH_4~+—N)尚未表现出显著变化,土壤硝态氮(NO_3~-—N)、土壤微生物量氮(MBN)均在HS处理下显著降低,且硫输入加剧土层间的差异性(P0.05);(4)硫输入抑制了酶活性,土壤脲酶活性在HS处理下显著降低(P0.05),土壤蔗糖酶活性无显著变化,但硫输入均弱化了土层间酶活性的差异性。综合分析所有处理下的土壤生化性质和酶活性等指标发现,对硫添加响应敏感的是土壤pH和酶,土层是另外一个主要影响因子,硫添加和土层的交互作用则影响有限。采用Pearson分析得出,硫输入改变了土壤生化特性、酶活性等指标间的相关性程度。可见,酸雨对土壤酸化的影响是一个逐渐累积的过程,外源性硫添加对土壤碳氮及酶活性的影响存在一定差异,这可为硫沉降环境胁迫下森林管理提供科学依据。     

Use of crop residues with alkaline slag to ameliorate soil acidity in an Ultisol

Information regarding the interaction between liming agents and crop residues on soil acidity amelioration is limited. A laboratory incubation study was undertaken to investigate the combined application of alkaline slag (AS, the major component is CaO) and crop residues with different C/N ratios and ash alkalinity content. Incorporation of amendments was effective in reducing soil exchangeable acidity and Al saturation and increasing exchangeable base cations (P < 0.05), but the effect of AS on soil pH adjustment was reduced when added with a high amount of residue with a low C/N ratio. Initial increases in soil pH were attributed to the release of alkalinity from the combined amendments and the mineralization of organic nitrogen (N). During subsequent incubation, the soil pH decreased because of nitrification. Crop residues with a high C/N ratio increased N immobilization and reduced net nitrification, resulting in a slight pH decrease. Crop residues with a low C/N ratio resulted in a sharp decrease in soil pH when applied with low levels of AS because of stimulated soil nitrification, whereas high AS had no consistent effect on net nitrification. Hence, compared to the control (pH = 4.21), a large increase in soil pH occurred, especially when peanut straw was applied at 10 g/kg (pH = 5.16). It is suggested that crop residues with high C/N ratio and also combined with a liming agent such as AS are preferred to ameliorate soil acidity. The liming effect of AS is likely to be negated if added in combination with residues with high N contents.     

外来入侵植物黄顶菊残体的恢复再生能力和在土壤中的分解特性

在华北平原, 外来入侵植物黄顶菊[Flaveria bidents (L.) Kuntze]是一种农田和生态环境中的恶性杂草, 使用除草剂清除势必威胁到农业生产尤其是蔬菜生产的安全性。本课题研究了黄顶菊的不同部位离体茎段和残留部分的恢复再生能力; 并采用土壤培养试验, 研究了含氮量为12.2 g·kg^-1的黄顶菊烘干残体和含氮量为23.3 g·kg^-1的白三叶草残体在土壤中的分解过程及矿质氮的供应能力, 探讨人工拨除的黄顶菊作为绿肥的可能性。试验表明, 黄顶菊不同部位离体茎段的再生能力达10%~80%, 不同大小的残留部分再生能力达70%~100%。因而人工除草后必须将拔除的黄项菊残体带出田间以防其再生。在整个60 d的土壤培养期间, 黄顶菊和白三叶草残体均表现为施入土壤的前10 d内表观CO₂-C日释放量最大, 以后趋于平稳; 但黄顶菊残体处理土壤的CO₂-C日释放量一直低于白三叶草。黄顶菊残体处理的土壤微生物量氮高于白三叶草残体处理及土壤对照; 微生物量碳则为黄顶菊残体处理低于白三叶草残体处理, 且与土壤对照相近。在培养的前20 d, 黄顶菊残体分解过程中显著固定了土壤矿质氮素, 培养40 d后的表现为矿质氮释放。黄顶菊残体具有提高土壤氮素营养和微生物量, 增肥土壤的作用, 人工拨除的黄顶菊经灭活处理后可以作为绿肥施入田间。     

Fortified cassava peel compost amendment for Amaranthus: Influence on plant growth,nutrients uptake and on soil nutrient changes

Nutrient supply through organic sources usually requires fortification for timely and optimum release of plant nutrients to achieve optimum crop performance. A pot experiment was conducted in a screen house to determine the optimum rate of cassava peel compost (CPC) fortification that supports optimum Amaranthus (Amaranthus cruentus L.) plant nutrient contents and residual soil nutrient contents. A compost of cassava peel and poultry manure was applied at 2.5; 5.0 and 7.5 t ha^?1each complemented with either 25 or 50 kg nitrogen (N), using nitrogen, phosphorus and potassium (NPK) 20-10-10 at 2 weeks before sowing Amaranthus. An unfertilized treatment served as control. Seeds were sown in plastic containers with a surface diameter of 24 cm filled with 5 kg soil, with a drain underneath. Seedlings were thinned to 4 plants/pot 2 weeks after planting. Plants were harvested at 5 weeks by ratooning and plant re-growth also harvested after 5 weeks. Soil pH was lower with high rates of 5.0 and 7.5 t ha^?1 CPC while the organic matter content was increased with increased CPC rate. Soil N was reduced but reflected in increased plant shoot and root N, with compost application. Soil P was generally increased but was not reflected in plant contents. Soil K contents were reduced and were reflected in increased plant contents. Application of 2.5 t ha^?1 CPC, fortified with either 25 or 50 kg N ha^?1 gave the optimum Amaranthus shoot nutrient contents with optimum residual soil nutrient contents.     

Organic anion-to-acid ratio influences pH change of soils differing in initial pH

Purpose

This study aimed to investigate the effect of initial soil pH and organic anion-to-acid ratio on changes in soil pH.

Materials and methods

Two soils (Podosol and Tenosol) along with two carboxylic acids (malic and citric acid) and their anions (sodium malate and citrate), commonly found in plant residues, were used in this study. Stock solutions of either malic acid and disodium malate or citric acid and trisodium citrate were added to pre-incubated soils at anion-to-acid ratios of 0:100, 10:90, 25:75, 50:50, 75:25, 90:10, 100:0 and at 0.25 g C kg^?1 soil. Soils were adjusted to 80 % field capacity and mixed thoroughly, and three replicates of 50 g of each soil were transferred into individual plastic cores and incubated at 25 °C in the dark for 30 days. Soil pH, respiration, NH₄ ⁺, and NO₃ ^? were determined.

Results and discussion

Soil pH increased linearly with increasing organic anion-to-acid ratio. The addition of organic anions to soil resulted in net alkalinisation. However, the addition of organic acids immediately decreased soil pH. During subsequent incubation, soil pH increased when the organic anions were decomposed. Alkalinity generation was lower in the Podosol (initial pH 4.5) than in the Tenosol (initial pH 6.2), and was proportional to anion-to-acid ratio across all the treatments. Cumulative CO₂-C release was approximately three times lower in the Podosol than the Tenosol at day 2 due to lower microbial activity in the low-pH Podosol.

Conclusions

Increasing anion-to-acid ratio of organic compounds increased soil pH. Increases in soil pH were mainly attributed to direct chemical reactions and decomposition of organic anions. Low pH decreased the amount of alkalinity generated by addition of organic compounds due to incomplete decomposition of the added compounds. This study implies that organic anion-to-acid ratio in plant residues plays an important role in soil pH change.     

土壤主要理化性质对湘粤污染农田镉稳定效果的影响

利用盆栽试验研究了稳定剂(石灰、海泡石联合施用)对湖南、广东两省区不同性质土壤上生长的小青菜(Brassica chinensis L.)生物量、重金属吸收以及土壤pH和重金属提取态含量的影响,探讨了影响镉(Cd)稳定修复效果的土壤性质参数。结果表明:施加稳定剂对增加酸性土壤上小青菜生物量效果显著,土壤pH、有机质(OM)、全量Cd和黏粒是影响小青菜生物量变化的主要因素;土壤pH、阳离子交换量(CEC)、OM、黏粒是影响小青菜Cd含量变化的主要因素;土壤pH、CEC、全量Cd和黏粒是影响土壤提取态Cd含量变化的主要因素。     

有机物料对强酸性茶园土壤的酸度调控研究

通过室内培养的方式,研究了不同添加剂量下,不同C/N与灰化碱含量的有机物料对酸性茶园土壤的改良能力.试验结果表明:有机物料的添加可以有效地减少土壤交换性酸、铝饱和度,增加土壤交换性碱基,但是在调节土壤pH能力上并非一定有效.初始阶段,“灰化碱”的释放与有机氮的矿化提高了土壤的pH,随后pH由于硝化作用出现不同程度的下降.C/N高的作物秸秆(小麦和水稻秸秆)能够有效地抑制硝化,使pH下降幅度较小;而C/N低的作物秸秆(花生秸秆和菜籽饼)促进硝化,使pH大幅度下降.最终土壤pH与其C/N呈正相关性(y=0.00343x+4.14,r=0.977),而与其灰化碱含量无关.并且随着秸秆添加剂量的加大,C/N高的作物秸秆最终调剂pH的能力是显著提高的(P＜0.05),而C/N低的作物秸秆最终调剂pH的能力没有显著提高(P＜0.05).因此,C/N高的作物秸秆可能更适合土壤酸度的长期调节,与其相关的田间试验需要进一步进行证实.     

施氮对田菁翻压还田滩涂盐渍土碳氮及细菌群落结构的影响

为探讨不同施氮处理下田菁翻压还田对滩涂盐渍土的改良效果,通过田间小区试验研究了不同施氮水平下(CK、SN1、SN2、SN3、SN4对应的施氮量分别为0、90、135、180、225 kg/hm²)绿肥田菁还田对土壤碳氮、pH、水溶性盐和细菌群落结构的影响。结果表明:SN3处理下田菁生物量和碳、氮累积量最高,分别为41 882、3 756和101.5 kg/hm²。作绿肥翻压还田后,则以SN2处理土壤有机碳、全氮含量最高,分别为6.51 g/kg和0.637 g/kg。各施氮处理下,田菁翻压后土壤微生物生物量碳氮含量低于CK处理。随田菁翻压量的增加,土壤pH呈逐步下降趋势,而土壤水溶性盐总量则随施氮水平和翻压量的增加而上升,但较种植前明显降低。不同施氮处理田菁翻压后,土壤中具有一定有机降解功能的变形菌门、酸杆菌门、拟杆菌门、放线菌门和绿弯菌门等细菌类群占据主导地位,且相对丰度随田菁翻压量的增加呈一定变化趋势,但细菌群落结构变化不明显。土壤门水平优势菌群相对丰度与土壤碳氮含量、pH和水溶性盐总量等指标均呈一定的相关关系,其中以拟杆菌门、厚壁菌门、...     

Cadmium bioavailability to Nicotiana tabacum L., Nicotiana rustica L., and Zea mays L. grown in soil amended or not amended with cadmium nitrate

Summary Mature (flowering) tobacco (Nicotiana tabacum cv. PBD6, Nicotiana rustica cv. Brasilia) and maize (Zea mays cv. INRA 260) plants were grown in an acid sandy-clay soil, enriched to 5.4 mg Cd kg^–1 dry weight soil with cadmium nitrate. The plants were grown in containers in the open air. No visible symptoms of Cd toxicity developed on plant shoots over the 2-month growing period. Dry-matter yields showed that while the Nicotiana spp. were unaffected by the Cd application the yield of Z. mays decreased by 21%. Cd accumulation and distribution in leaves, stems and roots were examined. In the control treatment (0.44 mg Cd kg^–1 dry weight soil), plant Cd levels ranged from 0.4 to 6.8 mg kg^–1 dry weight depending on plant species and plant parts. Soil Cd enrichment invariably increased the Cd concentrations in plant parts, which varied from 10.1 to 164 mg kg^–1 dry weight. The maximum Cd concentrations occurred in the leaves of N. tabacum. In N. rustica 75% of the total Cd taken up by the plant was transported to the leaves, and 81% for N. tabacum irrespective of the Cd level in the soil. In contrast, the Cd concentrations in maize roots were almost five times higher than those in the leaves. More than 50% of the total Cd taken up by maize was retained in the roots at both soil Cd levels. The Cd level in N. tabacum leaves was 1.5 and 2 times higher at the low and high Cd soil level, respectively, than that in N. rustica leaves, but no significant difference was found in root Cd concentrations between the two Nicotiana spp.Cd bioavailability was calculated as the ratio of the Cd level in the control plants to that in the soil or as the ratio of the additional Cd taken up from cadmium nitrate to the amount of Cd applied. The results showed that the plant species used can be ranked in a decreasing order as follows: N. tabacum > N. rustica > Z. mays.     

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     

The amelioration effects of low temperature biochar generated from nine crop residues on an acidic Ultisol

Biochar was prepared using a low temperature pyrolysis method from nine plant materials including non‐leguminous straw from canola, wheat, corn, rice and rice hull and leguminous straw from soybean, peanut, faba bean and mung bean. Soil pH increased during incubation of the soil with all nine biochar samples added at 10 g/kg. The biochar from legume materials resulted in greater increases in soil pH than from non‐legume materials. The addition of biochar also increased exchangeable base cations, effective cation exchange capacity, and base saturation, whereas soil exchangeable Al and exchangeable acidity decreased as expected. The liming effects of the biochar samples on soil acidity correlated with alkalinity with a close linear correlation between soil pH and biochar alkalinity (R² = 0.95). Therefore, biochar alkalinity is a key factor in controlling the liming effect on acid soils. The incorporation of biochar from crop residues, especially from leguminous plants, can both correct soil acidity and improve soil fertility.     

碱渣与菜籽饼共施对茶园土壤酸度调控的研究

Rapeseed cake (RC), the residue of rapeseed oil extraction, is effective for improving tea (Camellia sinensis) quality, especially taste and aroma, but it has limited ability to ameliorate strongly acidic soil. In order to improve the liming potential of RC, alkaline slag (AS), the by-product of recovery of sodium carbonate, was incorporated. Combined effects of different levels of RC and AS on ameliorating acidic soil from a tea garden were investigated. Laboratory incubations showed that combined use of AS and RC was an effective method to reduce soil exchangeable acidity and Al saturation and increase base saturation, but not necessarily for soil pH adjustment. The release of alkalinity from the combined amendments and the mineralization of organic nitrogen increased soil pH initially, but then soil pH decreased due to nitrifications. Various degrees of nitrification were correlated with the interaction of different Ca levels, pH and N contents. When RC was applied at low levels, high Ca levels from AS repressed soil nitrification, resulting in smaller pH fluctuations. In contrast, high AS stimulated soil nitrification, when RC was applied at high levels, and resulted in a large pH decrease. Based on the optimum pH for tea production and quality, high ratios of AS to RC were indicated for soil acidity amelioration, and 8.0 g kg-1 and less than 2.5 g kg-1 were indicated for AS and RC, respectively. Further, field studies are needed to investigate the variables of combined amendments.     

N-mineralization in polyphenol-rich plant residues and their effect on nitrification of applied ammonium sulphate

Soil was incubated under greenhouse conditions with plant residues having varying phenolic and nitrogen contents. The total plant material added in staggered applications every 4 months was 15 g kg^?1 soil and the total incubation period was 12 months.The N-mineralization in these plant residues as influenced by their phenol and N contents was examined. The nitrification of applied (NH₄)₂SO₄ in these amended soils was also investigated under optimum conditions of pH.A high plant-N content resulted in increased N-mineralization of plant residue, but this effect was lowered by the presence of high concentrations of polyphenols in the decomposing residue, most probably due to increased participation of N with polyphenols in the formation of humus fractions.Soils amended with phenol-rich residues did not show any inhibition of nitrification of applied (NH₄)₂SO₄. Possible reasons are discussed. In organic matter decomposition, the quality of the leaf polyphenols appears to determine the degree of inhibition to soil nitrification.     

Plant canopy effects on litter accumulation and soil microbial biomass in two temperate forests

The objective of this study was to determine whether differences in canopy structure and litter composition affect soil characteristics and microbial activity in oak versus mixed fir-beech stands. Mean litter biomass was greater in mixed fir-beech stands (51.9t ha⁻¹) compared to oak stands (15.7t ha⁻¹). Canopy leaf area was also significantly larger in mixed stands (1.96m² m⁻²) than in oak stands (1.73m² m⁻²). Soil organic carbon (C _org) and moisture were greater in mixed fir-beech stands, probably as a result of increased cover. Soil microbial biomass carbon (C _mic), nitrogen (N _mic), and total soil nitrogen (N _tot) increased slightly in the mixed stand, although this difference was not significant. Overall, mixed stands showed a higher mean C _org/N _tot ratio (22.73) compared to oak stands (16.39), indicating relatively low rate of carbon mineralization. In addition, the percentage of organic C present as C _mic in the surface soil decreased from 3.17% in the oak stand to 2.26% in the mixed stand, suggesting that fir-beech litter may be less suitable as a microbial substrate than oak litter.     

室内恒温条件下稻田土壤中菌渣的分解过程及CO2释放特征

菌渣是栽培食用菌后的下脚料,可作为有机肥再利用。本文通过实验室条件下培养不同比例的菌渣和稻田土壤混合物[不施用菌渣(TS),土壤与菌渣质量比为10∶1(SM1)、5∶1(SM2)和2∶1(SM3),全部菌渣(TM)],研究不同处理有机碳和全氮的变化,探讨菌渣在稻田土壤中的分解过程,并分析CO_2释放特征,为菌渣合理利用提供参考。结果表明,在相同培养时间,添加不同比例菌渣处理有机碳和氮含量均比TS处理高,其中TM处理的有机碳和全氮分别比TS处理提高了10.7倍和11.0倍。有机碳、氮含量的提高量主要依赖于菌渣的添加量。总体来说,各处理随培养时间的延长,由于碳氮的分解,有机碳、氮均有下降趋势;在35 d后TM处理有机碳氮下降较快。添加菌渣越多,有机碳残留率也越大。在培养63 d后,菌渣有机碳(YC)和氮(YN)的分解残留率与菌渣添加量(X)的关系式分别为:YC=71.26X-0.607 5,r2=1.000 0**和YN=74.039X-0.413 3,r2=0.999 9**。各处理土壤CO_2释放速率均表现出先增后降然后趋于稳定趋势。菌渣用量越高,CO_2释放速率越高,各处理在不同培养时间CO_2释放速率均表现为TMSM3SM2SM1TS。在第7 d时各处理CO_2释放速率最高,在第14 d时渐渐处于平稳下降状态,培养35 d后,各处理土壤有机碳矿化强度很小,大部分有机碳被固定在土壤中,其中TM处理有机碳矿化强度最小。总之,还田菌渣越多,土壤中被固定的碳越多。     

Model organic compounds differ in priming effects on alkalinity release in soils through carbon and nitrogen mineralisation

The influence of organic matter and its cycling on soil pH change is still unclear. This study investigated the effect of organic compounds on carbon and nitrogen dynamics and their relationship with pH changes in two soils differing in initial soil pH (Podosol of pH 4.5 and Tenosol of pH 6.2). Seven organic compounds representing common compounds in decomposing plant residues or root exudates were added to the soils and incubated for 60 d. The largest cumulative soil respiration occurred when glucose, malic acid and citric acid were added. In addition, the Tenosol had the greater respiration compared to the Podosol. The addition of organic acids (acetic, malic, citric, ferulic and benzoic acid) instantly decreased soil pH due to the dissociation of H⁺ from the acids. The pH of both soils was then restored over time, which was positively correlated with decomposition % of these compounds. The pH of the Tenosol amended with all the organic acids and of the Podosol with malic acid exceeded that of the control, and net alkalization occurred, with the degree of alkalization being greater with malic and citric acid. Adding organic acids to the Tenosol generally increased NH₄ concentrations but decreased NO₃ concentrations. The addition of glucose decreased pH in Podosol but slightly increased it in the Tenosol. The addition of glucosamine hydrochloride decreased pH due to significant nitrification. The results suggest that the addition of organic acids stimulates microbial NO₃ uptake, and ammonification and decomposition of indigenous soil organic matter, resulting in a priming effect on alkalinity release, and that the degree of the priming effect is influenced by the type of organic acid and initial soil pH.     

Decarboxylation of organic anions to alleviate acidification of red soils from urea application

Purpose

Decarboxylation of organic anions in crop straw is recognized as one of the mechanisms for increasing pH in acidified soils. However, the effectiveness of specific compounds in alleviating soil acidification from nitrification has not been well determined. This study examined three organic anions commonly found in crop straws and their effect on soil acidity and N transformation processes following urea application to a red soil (Ferralic Cambisol).

Materials and methods

A 35-day incubation experiment was conducted using soil after receiving 26 years of two different nutrient treatments: (1) chemical nitrogen, phosphorus, and potassium fertilization (NPK, pH 4.30) and (2) NPK plus swine manure (NPKM, pH 5.88). Treatments included three rates (0.25, 0.5, and 1.0 g C kg^?1) of calcium citrate, 0.5 g C kg^?1 calcium oxalate, 0.5 g C kg^?1 calcium malate, urea-only (control) soil, and a non-treated soil as a reference. Soil acidity, mineral N species, decarboxylation, and their correlations were determined.

Results and discussion

All three organic anions significantly increased pH in both soils and the effectiveness was positively correlated with application rate. The change in total exchangeable soil acidity was dominated by aluminum concentration in the NPK soil, but by proton concentration in the NPKM soil. At ≥?0.5 g C kg^?1, the anions decreased soil exchangeable acidity by 25–68% in NPK soil and by 63–88% in NPKM soil as compared with control. Oxalate was the most effective in increasing soil pH by 0.70 and 1.31 units and reducing exchangeable acidity by 3.79 and 0.33 cmol₍₊₎ kg^?1 in NPK and NPKM soils, respectively, and also resulted in the highest CO₂ production rate. Addition of organic anions led to a lower nitrification rate in NPKM soil relative to the NPK soil.

Conclusions

These results imply that crop straws rich in organic anions, especially oxalate, would have a higher potential to alleviate soil acidification.

     

Amelioration of an acid ultisol by agricultural by‐products

Ultisols are widely distributed in the subtropical regions of China as well as in the world. High acidity of Ultisols limits plant growth and reduces crop yields. Amelioration of an acid Ultisol was investigated by incorporating the residues of canola (Brassica campestris L.), wheat (Triticum aestivum L.), rice (Oryza sativa), corn (Zea mays), soybean (Glycine max), peanut (Arachis hypogaea), faba bean (Vicia faba L.) and pea (Pisum sativum) and Chinese milk vetch (Astragalus sinicus L.) shoots after incubation of the agricultural by‐products for a maximum of 75 days, soil pH was increased by each of the plant materials. The degree of amelioration of the soil acidity by the plant materials was found to depend on the ash alkalinity and N content of the materials; the legumes of higher ash alkalinities and lower N contents, such as peanut straw and faba bean straw, led to the largest increases in soil pH, while the legumes of higher N contents showed less amelioration of the acidity to a certain degree, because of the release of protons during nitrification of NH from mineralisation of organic N. The non‐leguminous materials have medium amelioration effects and increased soil pH by 0·42–0·56 units at the end of incubation. The incorporation of the plant materials also increased exchangeable base status and reduced exchangeable Al, and thus decreased the toxicity of Al in the soil. This study demonstrates that plant materials, especially crop residues, can be used as amendments for acidic soils to restore degraded land in subtropical regions. Copyright © 2010 John Wiley & Sons, Ltd.