Predicting environmental soil phosphorus limits for dissolved reactive phosphorus loss

The dependence of runoff dissolved reactive phosphorus (DRP) loss on soil test P or rapid estimations of degree of P saturation (DPS) often varies with soil types. It is not clear whether the soil‐specific nature of runoff DRP versus DPS is due to the different sorption characteristics of individual soils or the inability of these rapid DPS estimates to accurately reflect the actual soil P saturation status. This study aimed to assess environmental measures of soil P that could serve as reliable predictors of runoff DRP concentration by using soils collected from Ontario, Canada, that cover a range of chemical and physical properties. A P sorption study was conducted using the Langmuir equation  to describe amount of P sorbed or desorbed by the soil (Q_s, mg/kg) versus equilibrium P concentration (C, mg/L) in solution, where Q_max is P sorption maximum (mg/kg), k represents P sorption strength (L/mg), and Q₀ (mg/kg) is the P sorbed to soil prior to analysis. Runoff DRP concentration increased linearly with increasing DPS_sorp (i.e. the ratio of (Q₀ + Q_D)/Q_max) following a common slope value amongst soil types, while the P buffering capacity (PBC₀) at C = C₀ yielded a common change point, below which runoff DRP concentration decreased greatly with increasing PBC₀ compared to that above the change point, where C₀ and Q_D represent the equilibrium P concentration and amount of P desorbed, respectively. Both DPS_sorp and PBC₀ showed great promises as indicators of runoff DRP concentration.     

蔬菜种植年限对土壤磷素吸附解吸特性的影响

为揭示不同种植年限土壤磷的固定和释放机制,通过土壤磷的等温吸附、解吸试验研究种植年限分别为3～5年、15～20年、25～30年的黄棕壤0～5cm和5～20cm土层磷的吸附、解吸特性。结果表明:土壤磷的等温吸附曲线、吸附量-解吸量曲线分别与Langmuir方程(R2为0.8728～0.8436)、二次函数方程拟合良好(R2为0.9545～0.9970);随蔬菜种植年限延长,表层土壤磷最大吸附量(Qm)、磷最大缓冲容量(MBC)明显降低,而土壤磷吸附饱和度(DPS)和解吸率明显提高;种植年限15～20年、25～30年土壤磷的解吸率明显高于3～5年土壤。对表征土壤磷素吸附、解吸特性的主要因子如MBC及DPS等作相关分析发现,无定形铁铝含量的变化是影响土壤磷吸附解吸特性的主要因素。     

The impact of perennial cormorant colonies on soil phosphorus status

During the last decades, the population of cormorants in Northern Europe has grown rapidly due to protection. Their impact on vegetation has been recognized, as many trees containing cormorant colonies have collapsed, but their influence on the soil phosphorus (P) status and related ecological impacts have not been studied in details.In this study, total and plant available P (P_Total, P_Olsen) together with phosphate adsorption capacity and saturation (PAC, P_Sat) were measured in the soil at a reference area (control) and below two cormorant sub-colonies that differed in age and bird dropping density (indicated by number of used nests). One colony with totally 831 nests was 24 years old (Cormorant₂₄), while the age of the other colony with totally 181 nests was 12 years (Cormorant₁₂). Analyses of soil samples collected in four layers down to 80 cm showed strong decreases in P_Total and P_Olsen in the order: Cormorant₂₄ >> Cormorant₁₂ >> control and severe leaching of P into the subsoil below 80 cm at Cormorant₂₄ but not at Cormorant₁₂. Accordingly, in all four Cormorant₂₄ soil layers and in the upper three Cormorant₁₂ layers, P_Sat exceeded 0.25, which is considered the limit between retaining and leaking soils; P_Sat = P_Ox/PAC and PAC = 0.5 × (Al_Ox + Fe_Ox), i.e. half the sum of oxalate-extractable aluminium and iron. The importance of P_Olsen and P_Sat as indicators for P leaching was demonstrated as well as the strong impact that cormorants can have on the soil P status. This, in turn, will not only affect the ecosystem balance below the colonies but probably also threaten the water quality in nearby open waters as cormorant colonies are normally located on small lake islands or close to shallow bays.     

长期不同施肥红壤磷素特征和流失风险研究

为探索长期施肥对红壤磷素吸附固持的影响,分析不同施肥土壤磷流失风险及影响因素。在南方丘陵区红壤上开展了持续25年的长期定位试验,处理包括:不施肥(CK)、施氮肥(N)、施磷肥(P)、施钾肥(K)、施氮磷钾肥(NPK1)、施2倍量氮磷钾肥(NPK2)、单施有机肥(OM)和氮磷钾配施有机肥(MNPK)。研究了不同施肥下土壤全磷、Olsen-P、Mehlich1-P、CaCl2-P含量及磷吸持指数(PSI)、磷饱和度(DPS)的变化,探讨不同施肥处理土壤对磷的吸附和解吸特征,并分析了土壤磷指标与土壤有机碳、pH、CEC之间的关系。结果表明:长期施用化学磷肥有利于补充土壤磷素,特别是土壤全磷,并使Olesn-P和Mehlich 1-P有增加趋势,而对CaCl2-P影响不显著;施用化肥对DPS影响不显著,单施磷会降低PSI,低量氮磷钾提高了PSI,高量氮磷钾处理与对照差异不显著;长期施用有机肥(猪粪)土壤全磷增加,而Olsen-P、Mehlich 1-P和CaCl2-P则大幅累积, PSI显著降低, DPS显著增加。长期施用化肥处理土壤对新添加磷的吸附较强,长期施用有机肥降低了土壤对新添加磷的吸附;土壤全磷、Olsen-P、Mehlich1-P、CaCl2-P、PSI、DPS及最大吸附容量(Qm)与土壤pH、CEC、土壤总有机碳(TSOC)、土壤水溶性有机碳[冷水提取水溶性有机碳(CWSOC)和热水提取水溶性有机碳(HWSOC)]间相关性较高;土壤磷指标和土壤有机碳、pH、CEC指标之间存在典型相关关系,第1对和第2对典型变量的典型相关系数分别为0.997和0.951,达显著水平。研究表明,施用有机肥是调节土壤磷的供给和保持的重要措施,土壤水溶性有机碳和pH可能是反映红壤磷素供应和流失的关键指标。     

磷素吸附特性演变及其与土壤磷素形态、土壤有机碳含量的关系

在黄土旱塬区长期试验(1985-1997年)中,选取对照(不施肥,CK)、磷肥(P2O5.60.kg/hm2,P)、氮肥(N.120kg／hm2,N)、氮磷(N,120.kg／hm2,P2O5,60.kg/hm2,NP)、氮磷有机肥(N.120.kg/hm2,P2O560.kg/hm2,有机肥75.t／hm2,NPM),种植方式为冬小麦连作的5种有代表性的施肥处理,研究了石灰性土壤磷素吸附特性的演变及其与土壤磷素形态、土壤有机碳(SOC)含量的关系。结果表明,P素的最大吸附量(Qm),1997年对照(CK)、N处理比1985年分别提高了18％和14％;而P、NP和NPM处理分别降低了26％、13％和24％。吸附能常数(k值)随时间延长,对照和N处理相对稳定,P和NP处理呈升高趋势,而NPM处理有降低趋势。土壤磷素吸附饱和度(DPS)和零净吸附磷浓度(EPC0)对照和N处理随时间延长呈降低趋势,P、NP和NPM处理呈升高趋势。Qm与Ca8-P、Al-P存在极显著相关关系(P0.001),与Ca2-P、Pe-P存在显著相关关系(P0.05)。Ca2-P、有机磷含量变化与土壤DPS的相关性达到显著水平(P0.05)。EPC0只与有机磷间存在显著的相关关系(P0.05)。Qm、DPS和EPC0变化与SOC存在显著或极显著的线性相关关系(P0.001)。     

Predicting the changes in environmentally and agronomically significant phosphorus forms following the cessation of phosphorus fertilizer applications to grassland

Phosphorus (P) loss from soil can impair surface water quality. Losses from soil are related to soil P concentrations, but agronomic measures such as Olsen P do not in many cases predict the potential for P loss. One possible strategy to decrease P loss is to stop applying P fertilizers. We examined the changes in both agronomic (Olsen P) and environmental [water‐extractable P (WEP) and calcium chloride‐extractable P (CaCl₂‐P)] P tests, and the potential implications following a halt to P fertilizer application to four long‐term grassland field trials on different soil types. Exponential decreases in Olsen P and WEP concentration over time were observed in three of the four trials, but only in one trial for CaCl₂‐P. The rates of decrease in Olsen P (OP) and WEP concentration were best correlated with initial WEP (WEP_i) concentration and the quotient of Olsen P_i/P retention (PR, a measure of Al‐ and Fe‐oxides), respectively. The equation t = 1/(?0.035 × ln OP_i/PR ? 0.0455) × (ln WEP_t ? ln WEP_i) was used to predict the time (t) taken for WEP concentrations at the sites to decrease to 0.02 mg/L (WEP_t), which is proposed as a limit for dissolved reactive phosphorus in overland flow, and the result was 23–44 yr. Results from a similar equation for Olsen P predicted a quicker rate of WEP. A significant decline in dry matter (DM) yield was observed at one trial site. For this site, the rate of decline in DM yield was of a similar magnitude to the rate of decline in WEP concentration. This suggests that halting P fertilizer application to decrease P loss as measured by WEP concentration may decrease farm productivity. An alternative, more financially acceptable, strategy is required, such as a negative P balance while maintaining yields with N fertilizer, but further work is required to assess both the agronomic and environmental implications of this strategy.     

Transformation of added phosphorus to acid upland soils with different soil properties in Indonesia

Abstract

The transformation of added phosphorus (P) to soil and the effect of soil properties on P transformations were investigated for 15 acid upland soils with different physicochemical properties from Indonesia. Based on oxide-related factor scores (aluminum (Al) plus 1/2 iron (Fe) (by ammonium oxalate), crystalline Al and Fe oxides, cation exchange capacity, and clay content) obtained from previous principal component analyses, soils were divided into two groups, namely Group 1 for soils with positive factor scores and Group 2 for those with negative factor scores. The amounts of soil P in different fractions were determined by: (i) resin strip in bicarbonate form in 30 mL distilled water followed by extraction with 0.5 mol L^?1 HCl (resin-P inorganic (P_i) that is readily available to plant), (ii) 0.5 mol L^?1 NaHCO₃ extracting P_{i and} P organic (P_o) (P which is strongly related to P uptake by plants and microbes and bound to mineral surface or precipitated Ca-P and Mg forms), (iii) 0.1 mol L^?1 NaOH extracting P_i and P_o (P which is more strongly held by chemisorption to Fe and Al components of soil surface) and (iv) 1 mol L^?1 HCl extracting P_i (Ca-P of low solubility). The transformation of added P (300 mg P kg^?1) into other fractions was studied by the recovery of P fractions after 1, 7, 30, and 90 d incubation. After 90 d incubation, most of the added P was transformed into NaOH-P_i fraction for soils of Group 1, while for soils of Group 2, it was transformed into resin-P_i, NaHCO₃-P_i and NaOH-P_i fractions in comparable amounts. The equilibrium of added P transformation was reached in 30 d incubation for soils of Group 1, while for soils of Group 2 it needed a longer time. Oxide-related factor scores were positively correlated with the rate constant (k) of P transformation and the recovery of NaOH-P_i. Additionally, not only the amount of but also the type (kaolinitic) of clay were positively correlated with the k value and P accumulation into NaOH-P_i. Soils developed from andesite and volcanic ash exhibited significantly higher NaOH-P_i than soils developed from granite, volcanic sediments and sedimentary rocks. Soil properties summarized as oxides-related factor, parent material, and clay mineralogy were concluded very important in assessing P transformation and P accumulation in acid upland soils in Indonesia.     

Sorption of phosphorus in field-moist and air-dried samples from four weakly developed cultivated soil profiles

Sorption of phosphorus (P) in complete soil profiles in northern Europe is not adequately documented. I measured the sorption in genetic horizons of four cultivated soils (Inceptisols, Spodosol) in Finland using both field‐moist and air‐dried soil samples, fitted modified Freundlich equations (Q = a × I^b ? q) to the data, and presented the results in quantity/intensity (Q/I) graphs. Least‐squares‐estimates for the parameters of the modified Freundlich equation (a, b, q) were found to be imprecise measures of sorption. Values derived from the fitted equations (the amount of P sorbed at the P concentration of 2 mg litre^?1 and P buffering capacity at the same concentration) were more precise. Both were correlated with concentrations of oxalate‐extractable iron and aluminium. In all soils, there was a distinct difference in sorption between the fertilized Ap horizons and the subsurface horizons, which retained P strongly. Most of the sorption capacity was located in the B horizons at depths between 0.3 and 0.7 m. The results demonstrate the effects of soil‐forming processes and human impact on the sorption of P in the soils. Drying the samples prior to the sorption experiments altered the shape of the Q/I graphs. It increased dissolution of P at small P concentrations, sorption at large P concentrations, and the estimates for P buffering capacity. The effects of drying soil samples on the results and the imprecision of the parameters estimated with the modified Freundlich equation should be taken into account when interpreting results of Q/I experiments.     

有机肥对磷的吸附-解吸的影响及在红壤发育成的水稻土上的有效性

A field test with the traditional rotation of paddy rice/upland crop (wheat) was carried out on a paddy soil derived from red earth to elucidate the effect of organic manure on the phosphorus adsorption-desorption by soil and its P availability. Soil samples were taken from different treatments at rice harvesting stage and analysed. The isothermal adsorption of P by the samples fitted very well with Langmuir equation, and hence, the parameters in the equation, i.e., maximum adsorption (qm), constant related to bonding energy (k) and their product (k × qm) could be used as a comprehensive index to characterize the potential P adsorptivity of the soil. Organo-inorganic fertilization and organic manuring could decrease qm and k, while mineral P application had little effect on them. The isothermal desorption of P was significantly correlated with initially added and isothermally adsorbed P. Part of P added was fixed, which represented the P fixation capacity of soil, and organic manuring could obviously lower the P fixation. The content of soil available P had a significant negative correlation with qm, k and fixed P. It is concluded that organic manure could increase the P availability of paddy soil derived from red earth by decreasing qm, k, maximum buffering capacity (MBC=k × qm) and fixation capacity.     

INORGANIC SOIL PHOSPHORUS

Sixteen soils and 4 soil preparations were cropped exhaustively with ryegrass in the glasshouse and monocalcium phosphate potentials (½pCa+pH₂PO₄=1) were measured after each of 6 consecutive harvests. The amounts of phosphorus (Q) removed from the soils by ryegrass accounted for 95·1–96·6 per cent of the variance in 1 for 3 soils and 2 soil preparations (P < 0·001), for 88·4–93·7 Per cent of the variance for 6 soils and 2 soil preparations (0·001 < P < 0·01), for 71·6–82·6 per cent of the variance for 3 soils (0·01 < P < 0·05) and for insignificant amounts of the variance for 4 soils. Values of ΔI/ΔQ ranged from 7 × 10^–4 to 431 × 10^–4½pCa+pH₂PO₄/ppm P removed from soil. ΔI/ΔQ tended to decrease (i.e. the soils were more buffered) with increasing clay contents and with increasing amounts of NaHCO₃-soluble P and to increase (i.e. the soils were less buffered) with increasing amounts of CaCO₃. Variations in organic C did not significantly affect ΔI/ΔQ. The following equation accounts for 81 per cent of the variance in ΔI/ΔQ for all soils except those in equilibrium with octacalcium phosphate: ΔI/ΔQ× (10⁴) = 225·9–4·17(% clay)+8·01(% CaCO₃)–1·38(ppm NaHCO₃-soluble P).     

Seasonal variation in casting activity and in the amounts and release to solution of phosphorus forms in earthworm casts

Production of surface casts of earthworms (dominantly Allolobophora caliginosa) varied markedly during 1975. Casting activity was restricted to the April to September period and reached a maximum in early June. Seasonal variations in cast production were attributed to fluctuations in soil moisture and temperature, and in food supply. Although the total P content of casts was essentially constant throughout the year, the inorganic P content decreased from a maximum in May to a minimum in August. The progressive decrease in the amount of inorganic P between May and August is attributed to a reduction in the conversion of organic to inorganic P because of the lower microbial and phosphatase enzyme activity as a result of declining soil temperature. Seasonal variations in the release of inorganic P from casts to solution and in the amounts of rapidly-exchangeable P were more closely related to the seasonal pattern of cast production than to changes in the amount of inorganic P in the casts. In contrast, seasonal changes in the release of inorganic P to solution and in the amount of rapidly-exchangeable P in underlying soil were small. A greater amount of inorganic P was extracted by 0.5 m NaHCO₃ than by water from freshly-deposited casts collected at different times of the year, indicating that NaHCO₃ extracts some chemisorbed inorganic P from casts. Water-extractable inorganic P accounted for approximately 90% of the difference between inorganic P in casts and underlying soil. Consequently, essentially all of the additional inorganic P in casts is retained by a more-physical sorption type. The results obtained are discussed in terms of the likely effects of surface casting on the P cycle in a soil-plant system.     

Estimation of the phosphorus sorption capacity of acidic soils in Ireland

The test for the degree of phosphorus (P) saturation (DPS) of soils is used in northwest Europe to estimate the potential of P loss from soil to water. It expresses the historic sorption of P by soil as a percentage of the soil's P sorption capacity (PSC), which is taken to be α (Al_ox + Fe_ox), where Al_ox and Fe_ox are the amounts of aluminium and iron extracted by a single extraction of oxalate. All quantities are measured as mmol kg soil^?1, and a value of 0.5 is commonly used for the scaling factor α in this equation. Historic or previously sorbed P is taken to be the quantity of P extracted by oxalate (P_ox) so that DPS = P_ox/PSC. The relation between PSC and Al_ox, Fe_ox and P_ox was determined for 37 soil samples from Northern Ireland with relatively large clay and organic matter contents. Sorption of P, measured over 252 days, was strongly correlated with the amounts of Al_ox and Fe_ox extracted, but there was also a negative correlation with P_ox. When PSC was calculated as the sum of the measured sorption after 252 days and P_ox, the multiple regression of PSC on Al_ox and Fe_ox gave the equation PSC = 36.6 + 0.61 Al_ox+ 0.31 Fe_ox with a coefficient of determination (R²) of 0.92. The regression intercept of 36.6 was significantly greater than zero. The 95% confidence limits for the regression coefficients of Al_ox and Fe_ox did not overlap, indicating a significantly larger regression coefficient of P sorption on Al_ox than on Fe_ox. When loss on ignition was employed as an additional variable in the multiple regression of PSC on Al_ox and Fe_ox, it was positively correlated with PSC. Although the regression coefficient for loss on ignition was statistically significant (P < 0.001), the impact of this variable was small as its inclusion in the multiple regression increased R² by only 0.028. Values of P sorption measured over 252 days were on average 2.75 (range 2.0–3.8) times greater than an overnight index of P sorption. Measures of DPS were less well correlated with water‐soluble P than either the Olsen or Morgan tests for P in soil.     

Sorption of phosphorus by some surface soils from Quebec in relation to their properties

Abstract

Surface horizons from Podzolic and Gleysolic soils were collected in various parts of the province of Quebec, Canada, and equilibrated with various amounts of KH₂PO₄ in 0.01 M CaCl₂ for 48 hours. P sorption data conformed to the linear form of the Langmuir and Freundlich equations. P solubility isotherms showed evidence of hydroxyapatite formation in most samples studied, whereas equilibration solutions of only few samples were saturated with respect to either dicalcium phoshate dihydrate or octocalcium phosphate. These reaction products were associated to soil pH and levels of added phosphate. The average values of the Langmuir sorption maximum for these studied Gleysolic and Podzolic samples were 763 and 1096 μg/g respectively. These values were higher than those obtained by the segmented and modified Freundlich models.

Relationships between the soil characteristics and P sorption parameters were evaluated by regression analysis. Among all variables, oxalate‐extractable Fe plus Al content of the Podzolic samples and the ratio of oxalate—extractable Al to clay of the Gleysolic samples gave the best significant correlation coefficients. Furthermore, soil pH and various ratios such as pyrophosphate‐extractable Fe and Al, oxalate‐extractable Fe and organic matter to clay were found to be significantly correlated only with the P sorption parameters of the Gleysolic samples.     

The decomposition of cocoa leaves and their effect on phosphorus dynamics in tropical soil

Higher-yielding varieties of cocoa make heavier demands on phosphorus resources in soils and so it is important that the role of leaf litter in cycling P is understood. Fresh cocoa leaves and leaf litter were incubated moist with a soil inoculum for 80 days when between 16 and 33% of the mass was lost. Materials containing large amounts of P or incubated with added inorganic P initially decomposed more rapidly than those containing smaller amounts, indicating that decomposition was limited by lack of P. Fresh leaves had half of their P in an acid-soluble (0.1 m H₂SO₄) form, most of which was also water soluble, whereas in the litters about a third was acid-soluble. During incubation, P-rich materials showed an increase in the acid-soluble fraction and a decrease in water-soluble P. Litters with small concentrations of P simply lost P from the acid-soluble into the non-soluble organic fraction, and no water-soluble P remained after 80 days. –A soil from a cocoa-growing site fertilized with P contained almost four times as much biomass P as the non-fertilized control (30 and 8 mg kg^–1 soil, respectively), the amounts of bicarbonate-extractable P being 32 and 4 mg kg^–1. Soils from these and one other cocoa-growing site (8 mg kg^–1 biomass P, 7 mg kg^–1 bicarbonate-extractable P) were incubated either alone, with cocoa litter, or with cocoa litter plus inorganic P. In the soil that had the small amount of NaHCO₃-extractable P (4 mg kg^–1) addition of litter caused the biomass P to increase from 8 to 16 mg kg^–1 after 1 week's incubation, the increase being larger than the amount of P added in the litter, but in the other two soils biomass P was not increased. Addition of inorganic P had no effect on biomass P in any of the soils. –Decomposing litter may compete with the crop for P, but addition of fertilizer P may increase the rate of mineralization of organic P in the litter. Suitable management of fertilizer P should allow the rate of release of P from the litter to be adjusted to suit crop demands.     

Comparison of extractable phosphorus from two sulfuric acid methods with the Mehlich phosphorus test number one

Abstract

All mineral phosphates in soil dissolve more completely when HCl is mixed with H₂SO₄ than with the HCl alone. It was hypothesized that a new extracting solution of H₂SO₄ alone with the same ionic strength or the same acidity as the Mehlich P1 extractant would extract similar amounts of soil phosphorus (P) as the Mehlich P1 soil test. Thirty six acid soils from Alabama, Georgia, North Carolina, South Carolina, and Colorado were used in this study. These acid soils reflect wide ranges in parent materials, texture, pH, organic matter, and available soil P. They were analyzed for available soil P with the Mehlich P1 soil test and with the two H₂SO₄ methods: Method A has an extracting solution of same ionic strength (0.0875M) as the Mehlich P1 extractant, and Method B was an extracting solution of the same acidity (0.0375M) as the Mehlich P1 extractant. Correlations between the results of Mehlich P1 with Method A and Method B were 0.994 and 0.997, respectively. The measured test precision was <3.5% for all three methods. The new H₂SO₄ methods are simple and faster to conduct under routine operations than the original Mehlich P1 extractant, and because of the high correlations, the H₂SO₄ methods should predict crop response to P as well as the original Mehlich P1 extractant for acid soils.     

无定河粗泥沙产沙量的变化及其原因

 为了解决黄河治理中迫切需要解决的粗泥沙来源和数量问题,依据大量实测资料,运用统计方法对无定河粗泥沙产沙量进行研究。结果表明:无定河流域d>0.05 mm粗泥沙产沙量呈现出减少趋势,这种减少与年降水量、年沙尘暴频率的减少和水土保持面积的增大有一定的因果关系;估算出最大30d降雨量和年沙尘暴频率的变化对d>0.05mm粗泥沙产沙量变化的贡献率分别为66.5%和33.5%,即沙尘暴特征的变化对粗泥沙产沙量变化的贡献率相当于暴雨特征的变化贡献率的1/2。建立了粗泥沙年产沙量与最大30d降雨量、年沙尘暴频率和水土保持措施面积之间的回归方程,表明粗泥沙年产沙量随最大30d降雨量、年沙尘暴频率的减小而减小,随水土保持面积的增大而减小。因此,20世纪50年代以来暴雨的减弱、沙尘暴的减弱和水土保持措施面积的增大是无定河流域粗泥沙产沙量减少的原因。     

Carbon and phosphorus transformations during decomposition of pine forest floor with different phosphorus status

Summary Information on the mineralization of inorganic phosphate (P_i) from organically bound P (Po) during decomposition of forest floor and soil organic matter is vital for understanding P supply in forest ecosystems. Carbon (C) and phosphorus (P) fluxes were determined for forest floor samples from three Pinus radiata plots which had received no P (Control), 62.5 kg P ha^–1 (Low P) and 125 kg P ha^–1 (High P) 20 years before sampling. The P concentration of the forest floor samples had increased with fertilizer application, and the C:P ratio ranged between 585 and 1465. During a 9-week laboratory incubation 8.2–19.0% of the forest floor C was evolved as CO₂-C. The amount of CO₂ evolved from the forest floor of the Control plot was more than twice the amounts from the Low P and High P plots. There was little change in net P mineralization in the Control and Low P treatments throughout the incubation, but it increased slightly for the High P samples, suggesting a critical forest floor C:P ratio of 550 for net P mineralization. Changes in the ³²P-specific activities of the P_i and microbial P pools during incubation, and concurrent changes in microbial-³²P and ³²P_i, indicated internal P cycling between these pools. The rate of internal P cycling varied with forest floor quality, and was highest in the High P forest floor. The High P samples had microbial C:P ratios of 22 : 1 which remained constant during the incubation, suggesting the microorganisms had adequate P levels. Received: 2 July 1997     

Addition of phosphorus to soils with low to medium phosphorus retention capacities. I. effect on soil phosphorus sorption properties

Abstract

Twelve soils with low‐medium phosphorus (P) retention capacities were equilibrated for 3 months with soluble phosphate at a rate of 100 mg P kg^‐1 soil. The P sorption properties of these soils both with and without added P were studied, including equilibrium P concentration (EPCo), standard P requirement (SPR), soil P sorption capacity (b), maximum buffer capacity (MBC), and P sorption index (P‐SI). In general, the soils with no added P showed low values of all the above parameters. Oxalate extractable aluminum appeared to be the major responsible element for the control of P sorption in these soils. The addition of P to these soils had a considerable effect on their P sorption properties. The changes in EPCo were well correlated with P sorption index (r=0.80; p≤0.01 ). The EPCo values of the soils with and without added P were closely correlated to bicarbonate extractable P (P_0lsen) and calcium chloride extractable P (P_CaC12), with r=0.80, and r=0.99 (p≤0.001), respectively. Ninety percent of the variability in EPCo was explained by the corresponding variability in P_Olsen when a curvilinear relationship was adopted. The P sorption properties examined appear to be useful parameters to assess the environmental impact of soil P on the quality of surface waters.     

Effect of coffee husk and cocoa pods biochar on phosphorus fixation and release processes in acid soils from West Cameroon

Acid soil in West Cameroon has limited phosphorus (P) availability which limits plant growth. This is mainly because of low pH, high levels of exchangeable aluminium (Al) and iron (Fe) and fixation of P. In this study, acid soils, sampled in Bafang, were amended with biochar produced from coffee husks (CH) and cocoa pod husks (CP) at two different temperatures (350 and 550 °C) in other to evaluate the effect on the physicochemical properties of the acid soil and the effect on P sorption and desorption. The soil was amended with biochar at a rate of 0, 20, 40 and 80 g/kg and incubated for 7 and 60 days. Physicochemical properties of all soil–biochar samples were determined followed by sorption experiments and data fitted in the Langmuir and Freundlich isotherm models in other to evaluate soil P sorption capacity and its affinity to soil amended with biochar. Moreover, desorption studies were done to evaluate the availability of P in soil amended with biochar after sorption. The outcomes of this study reveal an increase in soil pH, electrical conductivity (EC), available P, soil organic carbon and a drastic decrease in exchangeable Al and Fe. The point of zero charge of biochar-amended soil was higher than the control and increased with amendment rate. The experimental data of the sorption of P on soils and soil–biochar samples fits into Langmuir and Freundlich models (R² > 0.9) suggesting that the P adsorption is controlled by both model mechanisms. Soil–biochar mixture results in a decrease in the sorption capacity as compared with the control and the decrease was predominant with increasing amendment rate. At amendment rates of 20, 40 and 80 g/kg after 7 days of incubation, $Q_{\max }$ for SCH350 were 2267, 2048 and 1823 mg/kg which increased to 2407, 2112 and 1990 mg/kg after 60 days of incubation. This tendency was observed for all biochar inputs with respect to the increase in incubation days. Furthermore, desorption of P from soil–biochar mixtures was enhanced with biochar added at greater rate and produced at higher temperature. The desorption percentage was increased by more than around 10% for all biochar types from 20 mg/kg to 80 mg/kg amendment. Thus, biochar addition to acid soils reduces P fixation to acid soil and improves P desorption to soil solution, thereby providing more available P in the soil solution and better conditions for plant growth.     

Effects of long-term phosphorus fertilization and winter cover cropping on soil phosphorus transformations in less weathered soil

Information concerning sources and sinks of available P in soil is needed to improve soil P management and protect water quality. This study, conducted from 1989 to 1998 on a Sultan silt loam soil (Aquantic Xerochrept), determined the annual P removal rate by corn (Zea mays L.) and P transformation as affected by P rate and winter cover cropping. Treatments included two P rates (0 and 44 kg P ha^–1) applied to corn at planting each year. All cover crops received 19.6 kg P ha^–1 at seeding each fall. Also included was a control without any cover crop and with no P addition. Corn yield and P uptake were affected by P fertilizer additions, but not by cover crops. A fairly constant amount of P was supplied from indigenous soil P when no external P was added. When the amount of P added exceeded that removed by corn, the excess P was converted mainly to NaOH-extractable inorganic P (NaOH-P_i). When the amount of P applied was below that removed by corn, indigenous soil NaOH-P_i acted as a source of available P for the plant. With no reduction of organic P (P_o) extractable by NaOH or NaHCO₃, the contribution from P_o to the available P pool appeared limited. The role of NaOH-P_i in P availability in the soil was substantiated by its significant correlation with labile NH₄Cl-extractable P (NH₄Cl-P; r² =0.60, P <0.001) or NaHCO₃-P_i (r² =0.81, P <0.001) pools. The NaOH-P_i for the soil reflected the changes in soil P resulting from past fertilizer P input and P removal by the crops.Scientific Paper Number 0005-34