Assessing the bioavailability of dissolved organic phosphorus in pasture and cultivated soils treated with different rates of nitrogen fertiliser

A proportion of dissolved organic phosphorus (DOP) in soil leachates is readily available for uptake by aquatic organisms and, therefore, can represent a hazard to surface water quality. A study was conducted to characterise DOP in water extracts and soil P fractions of lysimeter soils (pasture before and after, and cultivated soil after leaching to simulate a wet winter-autumn) from a field trial. Data on DOP in drainage waters from the field trial were also generated. In water extracts, used as a surrogate for soil solution and drainage water, 70-90% of the total dissolved P (TDP) concentration was made up of DOP, of which 40% was hydrolysable by phosphatase enzymes. Proportions of hydrolysable DOP to TDP in drainage waters of the field trial were less than in water extracts due to enhanced DRP loss via dung inputs, but still large at 35% of DOP. Analysis of lysimeter soils by sequential fractionation indicated that several organic P fractions changed with land use and due to leaching. Further investigation using NaOH-EDTA extracts and ³¹P nuclear magnetic resonance spectroscopy indicated that the greatest changes were a decrease in the concentrations of orthophosphate diester P and an increase in orthophosphate monoester P. This was attributed to mineralization by cultivation and plant roots and also to the leaching of mobile diester P. This study suggests that in such soils with a dynamic soil organic P pool, the concentration of readily bioavailable P in soil solution and drainage waters and the potential to impair surface water quality cannot be determined from the DRP concentration alone.     

Phosphorus retention and release of anions and organic carbon by two Andisols

Andisols can absorb large amounts of phosphorus rapidly, and then release it slowly, yet the mechanisms by which they retain P and release it for plant growth are poorly understood. Ligand exchange of organic compounds from Al–humic complexes by P and/or Si release – due to breakdown of allophanic microstructure to provide sorption sites – might account for the retention of P, but its extent is not known. We applied a soil column flow-through technique to quantify the release of anions and organic carbon (C) associated with P sorption by two andic soils, and we related the anion release to possible mechanisms for the retention of P. Phosphate (H₂PO₄^–, HPO₄^2–) sorption and concurrent anion desorption were obtained by passing a 1-g P 1^–1 (32 mmol KH₂PO₄ in 1 mm CaCl₂) solution through the soil columns (25 cm³). Total dissolved P, Fe, Al, S, Ca, Mg, K, Mn, organic C and pH were determined in the eluent. Changes in eluent pH and the patterns of the retention of P and corresponding concentrations of Al, Si and organic C in the eluent were similar for the two Andisols. The general pattern and changes in pH of the eluent coincided with changes in the patterns of release of organic C and Si and the rate of P retention. Release of silica accounted for < 6% of the P sorbed and had only a minor role in P retention in these two Andisols. Release of organic C, however, accounted on a molar basis for 40% and 83%, respectively, of the P sorbed. Direct measurements of the pH of the eluent and release of anions and organic C concurrent to P retention contribute to rapid assessment of the controlling mechanisms of P retention. The results indirectly confirm the hypothesis of ligand exchange of solution P with organic complexes held on allophanic surfaces. The organic C release, however, is not specifically related to either the fast or the slow P retention phase. The shift in the controlling P retention reaction associated with a change from the fast to the slow P retention phase is clearly indicated by an abrupt change of the pH of the eluent. This shift, in previous studies identified graphically by a change in slope of the P sorption isotherm, can be identified directly by measuring the pH of the matrix.     

Assessment of phosphate solubility in forest and highly fertilized Andisols and Andic soils

Abstract

Phosphate solubility in Andisols and Andic soils (forest and fertilized) has been studied in soil solutions at two soil:solution ratios (1:2.5 and 1:25). Forest soils approached the variscite solubility (pIAP=30.4±0.3) while fertilized soils were closer to the amosphous analog of variscite (pIAP= 29.8±0.2). In some samples, phosphorus (P) activities were consistent with simultaneous equilibrium between aluminum (Al) and iron (Fe) phosphates. The dilution ratio, DR=10(P)_1:25/(P)_l:25, was 10.0±0.4 for all samples. This suggests a dissolution of phosphate minerals attaining apparent saturation in 24 h. However, the high content of dissolved organic carbon and Al in aqueous extracts also pointed to the existence of soluble ternary complexes of P with Al(Fe)‐humus complexes. These results could be consistent with simultaneous control between the dissolution of variscite‐like minerals and humus‐Al(Fe)‐P associations. Nevertheless, DR seems a valuable criterion to assess soil P status, especially in fertilized soils with high active Al and Fe contents.     

The phosphorus composition of soil solutions and soil leachates: Influence of soil:solution ratio

Compositional differences between soil solutions obtained by different methods have frequently been reported; variations in the soi1: solution ratio may explain these results. In this study we compared the amount and composition of phosphorus (P) in soil leachates and soil solutions from a temperate grassland soil in northeast Scotland and determined the influence of soi1:solution ratio on P fractions in soil water extracts. Leachates were collected from intact soil cores over 6 months, the cores were then destructively sampled, and soil solutions obtained by centrifuging. Molybdate reactive P (MRP) represented 71% of the total dissolved P (TDP) in soil leachates but only 54% in soil solutions. The MRP component in soil water extracts increased from 71% to 92% as the soi1:solution ratio increased from 1:15 to 1:15·4, while the dissolved organic P (DOP) component decreased from 26% to 6%. As the soil:solution ratio increased the amount of MRP extracted increased; by contrast the amount of DOP and dissolved condensed P (DCP) extracted remained constant. While the MRP component is regulated by soil sorption processes, the supply and amount of DOP and DCP is probably related to biological activity. Dissolved organic carbon (DOC) extracted at wide soi1:solution ratios contained a smaller proportion of P than that extracted at narrower ratios. The results indicate differences in the behaviour of P fractions in the soil at various soi1:solution ratios and that these are reflected in the P composition of soil solution and leachate.     

Responses of soil dissolved organic matter to long-term plantations of three coniferous tree species

Tree species have significant effects on the availability and dynamics of soil organic matter. In the present study, the pool sizes of soil dissolved organic matter (DOM), potential mineralizable N (PMN) and bio-available carbon (C) (measured as cumulative CO₂ evolution over 63 days) were compared in soils under three coniferous species — 73 year old slash (Pinus elliottii), hoop (Araucaria cunninghamii) and kauri (Agathis robusta) pines. Results have shown that dissolved organic N (DON) in hot water extracts was 1.5–1.7 times lower in soils under slash pine than under hoop and kauri pines, while soil dissolved organic C (DOC) in hot water extracts tended to be higher under slash pine than hoop and kauri pines but this was not statistically significant. This has led to the higher DOC:DON ratio in soils under slash pine (32) than under hoop and kauri pines (17). Soil DOC and DON in 2 M KCl extracts were not significantly different among the three tree species. The DOC:DON ratio (hot water extracts) was positively and significantly correlated with soil C:N (R² = 0.886, P < 0.01) and surface litter C:N ratios (R² = 0.768, P < 0.01), indicating that DOM was mainly derived from litter materials and soil organic matter through dissolution and decomposition. Soil pH was lower under slash pine (4.5) than under hoop (6.0) and kauri (6.2) pines, and negatively correlated with soil total C, C:N ratio, DOC and DOC:DON ratio (hot water extracts), indicating the soil acidity under slash pine favored the accumulation of soil C. Moreover, the amounts of dissolved inorganic N, PMN and bio-available C were also significantly lower in soils under slash pine than under hoop and kauri pines. It is concluded that changes in the quantity and quality of surface litters and soil pH induced by different tree species largely determined the size and quality of soil DOM, and plantations of hoop and kauri pine trees may be better in maintaining long-term soil N fertility than slash pine plantations.     

Phosphorus Forms in Overland Flow from Agricultural Soils Representative of Mediterranean Areas

Abstract

Phosphorus (P) losses through overland flow (surface runoff) may contribute to eutrophication of water bodies. The main purpose of this work was to study P forms in overland flow (dissolved and particulate) to identify which can be potentially used by algae. To this end, rainfall on 17 representative soils from Mediterranean areas was simulated, and P forms in overland flow studied by chemical and sink (resin, iron oxide–impregnated paper strip) extraction; sequential chemical fractionation of the suspended sediments was also used to establish “operational pools” with a differential capacity of P release. Total P (TP) in runoff ranged from 0.089 to 0.765 mg L^?1 and was mainly related to suspended sediment (particulate P, 86% of TP on average). Iron oxide strip P, which is taken to be an estimate of algal‐available P, accounted for 34% of TP on average in runoff samples; most of the P extracted by this sink was particulate P (68%). In most cases, FeO strip P was equivalent to dissolved reactive P (DRP) plus P extracted by NaOH and citrate‐bicarbonate in suspended sediment (the more labile P fractions) (Y=X, R²=0.82; P<0.001; n=15). One can thus assume that Fe oxide extracts DRP, adsorbed P on sediments, and P related to highly soluble precipitated Ca phosphates, but it does not extract releasable P through reduction of sorbent surfaces or the organic P that can be mineralized in the bottom of water reservoirs, which must be taken in account to estimate the long‐term algal‐available P in runoff.     

Microbial decomposition of leached or extracted dissolved organic carbon and nitrogen from pasture soils

Microbial decomposition of extracted and leached dissolved organic carbon (DOC) and nitrogen (DON) was demonstrated from three pasture soils in laboratory incubation studies. DOC concentration in water extracts ranged between 29 and 148 mg C L^?1 and DON concentration ranged between 2 and 63 mg N L^?1. Between 17 and 61 % of the DOC in the water extracts were respired as CO₂ by microbes by day 36. DON concentrations in the extracts declined more rapidly than DOC. Within the first 21 days of incubation, the concentration of DON was near zero without any significant change in the concentration of NO₃ ^? or NH₄ ⁺, indicating that microbes had utilized the organic pool of N preferentially. Decomposition of leached DOC (ranged between 7 and 66 mg C L^?1) and DON (ranged between 6 and 11 mg N L^?1) collected from large lysimeters (with perennial pasture; 50 cm diameter?×?80 cm deep) followed a similar pattern to that observed with soil extracts. Approximately 28 to 61 % of the DOC in leachates were respired as CO₂ by day 49. The concentration of DON in the leachates declined to below 1 mg N L^?1 within 7–14 days of the incubation, consistent with the observations made with extractable DON. Our results clearly show that DOC and DON components of the dissolved organic matter in pasture soils, whether extracted or leached, are highly decomposable and bioavailable and will influence local ecosystem functions and nutrient balances in grazed pasture systems and receiving water bodies.     

Release and mobility of polycyclic aromatic hydrocarbons and iron‐cyanide complexes in contaminated soil

Export of organic and inorganic pollutants from the unsaturated zone of manufactured gas plant (MGP) sites may endanger groundwater quality. The release and mobility of polycyclic aromatic hydrocarbons (PAH) and iron‐cyanide complexes (FeCN) in soils from a former MGP site were studied in column and batch experiments. The unsaturated column experiment involved two parts. (1) The development of steady‐state effluent concentrations was studied under constant irrigation. (2) Rate‐limited contaminant release was studied by the system's response to flow interruption. Transport data were supplemented by a sequential batch‐desorption experiment. During constant irrigation the initial concentrations of total PAH and FeCN, but also the concentration of dissolved organic carbon (DOC) were higher than the steady state level, indicating that a specific fraction of the DOC and the contaminants was readily mobilized. Cyanide reached a constant effluent concentration after seven pore volumes (pvs), whereas equilibration of effluent PAH took 20 pvs. After flow interruption, rate‐limited release of DOC, PAH, and FeCN was observed by increased effluent concentrations. Comparison of FeCN levels with theoretical equilibrium concentrations indicated that in addition to the dissolution of iron(II)ferrocyanide (Berlin Blue), Fe₄[Fe(CN)₆]₃, other cyanide‐species may control the release and such the mobility of FeCN. This was confirmed by decreasing FeCN concentrations during sequential batch extractions. Due to rate‐limited release, seepage water concentrations of DOC, PAH, and FeCN in alkaline MGP soils are influenced by the flow regime. Experiments performed at continuous irrigation may therefore severely underestimate the release of PAH and FeCN under field conditions. The effect of DOC on the release and mobility of PAH and FeCN under transient conditions has to be further investigated.     

Dissolved reactive phosphorus in a Calcaric Fluvisol as affected by the addition of agricultural wastes

Abstract. Land application of organic wastes can result in the accumulation of available soil phosphorus (P) and in an increase risk of eutrophication of surface and shallow groundwaters. We conducted laboratory experiments to examine the effect of waste application on the concentration of dissolved reactive phosphorus (DRP) in 1:5 soil to 0.005 m CaCl₂ suspensions. Ten organic wastes, of which eight were mature composts, were applied to a P-rich Calcaric Fluvisol at rates ranging from 0 to 10% of soil by weight, and the difference in DRP concentration between suspensions containing waste and the untreated control (ΔDRP) was measured over a period of 300 days. In half of the suspensions of each waste–soil combination, 80 mg P (as KH₂PO₄) kg⁻¹ soil was applied at day 14. Values of ΔDRP were generally positive, but a significant number of negative values were also recorded for some wastes and application rates, particularly at later sampling times if inorganic P had been added. Regression analyses revealed that ΔDRP (i) increased with increasing soluble reactive P at all times and (ii) increased in the short term, but decreased in the long term with increasing water soluble organic carbon in the waste. The fact that ΔDRP was sometimes negative for some waste types suggests that application of organic waste to soil does not necessarily increase eutrophication risks caused by soil P losses.     

Eluviation of dissolved organic carbon under wetting and drying and its influence on water infiltration in degraded soils restored with vegetation

One mechanism in the restoration of severely degraded soil by vegetation might be the movement of dissolved organic carbon (DOC) to macropore and aggregate surfaces. We propose that this lowers the soil wetting rate and subsequently its slaking resistance by creating a partially hydrophobic surface. In this study, we determined how wetting and drying (w/d) cycles redistribute DOC to soil surfaces, and how DOC affects hydrophobicity where it accumulates, in relation to the soil surface area to volume ratio and to different types of vegetation planted to restore a severely degraded soil. Repacked soil cores that simulate different soil aggregate sizes were tested. The results showed that w/d cycles increase surface DOC concentration through a depletion of DOC in the interior of the soil. Correspondingly, w/d cycles enhanced hydrophobicity, measured as a water repellency index, R, from 1.5–2.3 to 3.6–7.6, the values affected significantly by the type of vegetation. This index (R) did not change for a control soil with no vegetation. The link between the amount of DOC and water repellency was weak (coefficient of determination r² = 0.06–0.26), indicating that DOC quality was probably more important than its quantity. Although increasing the core size resulted in a greater accumulation of DOC on the drying surface of the core, the impact of this on water repellency was minimal. Incubation caused a decrease in the amount of DOC, but had minimal influence on water repellency. This work improves the understanding of changes in soil wetting and soil stabilization under processes of natural weathering and vegetation restoration.     

The Effectiveness and Feasibility of Using Ochre as a Soil Amendment to Sequester Dissolved Reactive Phosphorus in Runoff

Incidental losses of dissolved reactive phosphorus (DRP) to a surface waterbody originate from direct losses during land application of fertilizer, or where a rainfall event occurs immediately thereafter. Another source is the soil. One way of immobilising DRP in runoff before discharge to a surface waterbody, is to amend soil within the edge of field area with a high phosphorus (P) sequestration material. One such amendment is iron ochre, a by-product of acid mine drainage. Batch experiments utilising two grassland soils at two depths (topsoil and sub-soil), six ochre amendment rates (0, 0.15, 1.5, 7.5, 15 and 30 g kg⁻¹ mass per dry weight of soil) and five P concentrations (0, 5, 10, 20 and 40 mg L⁻¹) were carried out. A proportional equation, which incorporated P sources and losses, was developed and used to form a statistical model. Back calculation identified optimal rates of ochre amendment to soil to ameliorate a specific DRP concentration in runoff. Ochre amendment of soils (with no further P inputs) was effective at decreasing DRP concentrations to acceptable levels. A rate of 30 g ochre kg⁻¹ soil was needed to decrease DRP concentrations to acceptable levels for P inputs of ≤10 mg L⁻¹, which represents the vast majority of cases in grassland runoff experiments. However, although very quick and sustained metal release above environmental limits occurred, which makes it unfeasible for use as a soil amendment to control P release to a waterbody, the methodology developed within this paper may be used to test the effectiveness and feasibility of other amendments.     

Sediment flow behavior in agro-watersheds of the purple soil region in China under different storm types and spatial scales

Conservation tillage has proved to be an effective option to reduce erosion and particulate phosphorus (PP) losses from fields to watercourses. However, it usually leads to the higher losses of P in dissolved reactive (DRP) form if P accumulates into the uppermost soil layer.In this study we investigated the influence of CaCO₃-rich boardmill sludge (BMS, applied 7 years ago) on soil properties and percolate quality with soil samples as well as undisturbed soil columns taken from a long-term conservation-tilled clay field experiment. A short-term laboratory experiment was undertaken to evaluate whether newly added BMS (FS) can be used to improve the quality of percolate waters by reducing turbidity, PP and DRP.The hypothesis was that BMS reduces the risk of dissolved reactive phosphorus (DRP) leaching from conservation-tilled soil. BMS served as a liming agent and counteracted the no-till-induced acidification and accumulation of DRP in the surface soil layer. Surface soil pH was on average 0.3 units higher and soil Ca 500 mg kg⁻¹ higher in the BMS-treated plots than in the plots without BMS. The ability of BMS to enhance the availability of P for plants may prevent the surface accumulation of P in NT cultivation. At the same time, BMS-induced increase in Ca and EC of the surface soil suppress the diffuse double layer by reducing P desorption and leaching risk.In the laboratory experiment, FS reduced percolate turbidity by 80% and PP and DRP concentrations by 62% and 50%, respectively. FS probably improved stability of soil aggregates in the laboratory experiment, obviously through enhanced microbial activity. The covering effect and infiltration of water through the 0–5 cm soil layer seemed to be crucial factors in reducing P losses.BMS proved to be a promising material for the applications to the conservation-tilled fields. The effect of BMS on microbes and immobilization of nutrients, should, however, be studied further in fields under a wider diversity of conditions.     

Fractionation of dissolved organic carbon in soil water: Effects of extraction and storage methods

Abstract

We measured the concentration and composition (sensu Leenheer, 1981) of dissolved organic carbon (DOC) in lysimeter solutions from the forest floor of a spruce stand in Maine and in laboratory extracts of organic (Oa horizon) and mineral soils collected from various forests in Maine, New Hampshire, and Vermont. All soils were acid Spodosols developed from glacial till. The effects of different storage, extraction and filtration methods were compared. Extracts from Oa horizons stored fresh at 3°C contained a larger fraction of hydrophobic neutrals than lysimeter forest floor solutions (31 and 4% of DOC in stored and lysimeter solutions, respectively), whereas extracts from Oa horizons which had been extracted, incubated at 10–15°C, and extracted again had DOC compositions similar to that in lysimeter solutions. Mechanical vacuum and batch extractions of Oa horizons yielded DOC similar in concentration and composition if the extracts were filtered through glass fiber filters. Nylon membrane filters, however, removed more hydrophobic acids from batch extracts. Dissolved organic carbon extracted from frozen, air‐dry, and oven‐dry Oa and Bh horizons was relatively rich in hydrophilic bases and neutrals and was similar to that released after chloroform fumigation, indicating that common soil‐storage methods disrupt microbial biomass.     

畜禽粪便中磷释放运移特征

随着畜禽粪便农田施用量的增加,畜禽粪便中磷流失也越来越引起人们的重视。采用取样分析和室内土柱模拟的方法,研究了猪粪和鸡粪中磷在水、0.5mol·L^-1NaHCO3和土体中的释放运移特点。结果表明,猪粪经H2O和NaHCO3连续提取后,H2O提取液中无机磷（P）i占猪粪全磷（TP）的21.58%,NaHCO3提取液中Pi占猪粪TP的28.92%;鸡粪经H2O和NaHCO3连续提取后,H2O提取液中Pi占鸡粪TP的18.09%,NaHCO3提取液中Pi占鸡粪TP的17.88%;施用猪粪和鸡粪处理土柱淋溶液中水溶性总磷（TDP）、水溶性无机磷（DRP）和水溶性有机磷（DOP）浓度均随着淋溶次数的增加呈先上升后降低的趋势,施用猪粪处理磷释放速率比施用鸡粪处理快;大量施用猪粪和鸡粪后,0～30cm土体中土壤全磷（TP）和0～60cm土体中土壤速效磷（Olsen-P）的含量显著增加。     

Temperature, water content and wet-dry cycle effects on DOC production and carbon mineralization in agricultural peat soils

Agricultural peat soils in the Sacramento-San Joaquin Delta, California have been identified as an important source of dissolved organic carbon (DOC) and trihalomethane precursors in waters exported for drinking. The objectives of this study were to examine the primary sources of DOC from soil profiles (surface vs. subsurface), factors (temperature, soil water content and wet-dry cycles) controlling DOC production, and the relationship between C mineralization and DOC concentration in cultivated peat soils. Surface and subsurface peat soils were incubated for 60 d under a range of temperature (10, 20, and 30 °C) and soil water contents (0.3-10.0 g-water g-soil⁻¹). Both CO₂-C and DOC were monitored during the incubation period. Results showed that significant amount of DOC was produced only in the surface soil under constantly flooded conditions or flooding/non-flooding cycles. The DOC production was independent of temperature and soil water content under non-flooded condition, although CO₂ evolution was highly correlated with these parameters. Aromatic carbon and hydrophobic acid contents in surface DOC were increased with wetter incubation treatments. In addition, positive linear correlations (r²=0.87) between CO₂-C mineralization rate and DOC concentration were observed in the surface soil, but negative linear correlations (r²=0.70) were observed in the subsurface soil. Results imply that mineralization of soil organic carbon by microbes prevailed in the subsurface soil. A conceptual model using a kinetic approach is proposed to describe the relationships between CO₂-C mineralization rate and DOC concentration in these soils.     

Phosphorus Availability and Some Biological Properties in the Bean (Phaseolus vulgaris) Rhizosphere

The objectives of this research were to evaluate the rhizospheric effects of bean (Phaseolus vulgaris) on phosphorus (P) availability and dissolved organic carbon (DOC), microbial biomass carbon (MBC), microbial biomass phosphorus (MBP), alkaline phosphatase (ALP) and P in the particulate fraction (PF-P) in some calcareous soils under rhizobox conditions. The results showed that DOC, MBC, MBP and ALP strongly increased in the rhizosphere soils compared with the bulk soils (p < 0.05). Also, the amounts of PF-P and P extracted with different tests in the rhizosphere were lower compared to the bulk soils (p < 0.05). The correlation studies showed that plant indices (dry yield and P uptake) had a positive relationship with Olsen-P, MBP, DOC, ALP and PF-P in both the rhizosphere and the bulk soil. Therefore, bean rhizosphere caused increases of DOC, MBC, MBP and ALP and decreases of available P and PF-P in the studied soils. In addition, the results of this research showed that the Olsen-P method and MBP and PF-P could be used to estimate bean-available P in the studied calcareous soils.     

养殖池塘底泥磷酸酶活性与释磷关系及其调控的研究

以天津市东丽区和西青区2个养殖池塘为对象,采用室内试验方法,研究了底泥中各有机磷组分与上覆水中可溶性磷（DRP）含量、底泥磷酸酶活性与有机磷各组分及上覆水中可溶性磷之间的关系,并采取不同处理对底泥酸性磷酸酶（ACP）和碱性磷酸酶（ALP）活性进行调控。结果表明,底泥中活性有机磷（LOP）、中等活性有机磷（MLOP）、中稳性有机磷（MROP）与上覆水中DRP之间呈显著正相关。底泥中ALP与MLOP、MROP之间相关显著,并对其直接影响较大;ACP与MLOP之间相关显著,对其产生的直接影响也很大。A、B两池塘底泥中ALP与上覆水中DRP随时间变化规律基本一致,二者之间呈显著正相关。在养殖水体中加入酶抑制剂和沸石能抑制ALP和ACP活性,并以酶抑制剂加沸石的处理效果最好。     

Assessment of a laboratory method to obtain the equilibrium solution composition of forest soils

Equilibrium studies on soil require reliable estimates of ion concentrations in the soil solution under field conditions. We evaluated the previously described iterative method to approximate the equilibrium soil solution (ESS) with four acid forest soils. We examined for which ions the ESS is suitable, making use of the fact that concentrations in water extracts are functions of the soil: solution ratio. The electric conductivity, pH, and the concentrations of base cations, Mn²⁺, NO^?₃, SO₄²⁺, and dissolved organic carbon (DOC) were usually linear with the soil:solution ratio in water extracts, whereas no relation was observed for Al (with one exception) and Fe. Assuming that the ESS can be attributed the soil solution ratio of the field moist soil at the time of sampling, concentrations appeared as the continuation of the linear relation with the soil: solution ratio for base cations, pH, and the electric conductivity. This indicates that the ESS actually represents field conditions for these solution properties. For Al water extracts allowed no evaluation of the ESS result. The ESS underestimated SO₄^2? concentrations under field conditions, presumably because the lack of DOC in the solutions added distorts the balance amongst anions.     

Dissolved organic carbon and nitrogen in precipitation,throughfall, soil solution,and stream water of the tropical highlands in northern Thailand

Dissolved organic matter (DOM) is important for the cycling and transport of carbon (C) and nitrogen (N) in soil. In temperate forest soils, dissolved organic N (DON) partly escapes mineralization and is mobile, promoting loss of N via leaching. Little information is available comparing DOC and DON dynamics under tropical conditions. Here, mineralization is more rapid, and the demand of the vegetation for nutrients is larger, thus, leaching of DON could be small. We studied concentrations of DOC and DON during the rainy seasons 1998–2001 in precipitation, canopy throughfall, pore water in the mineral soil at 5, 15, 30, and 80 cm depth, and stream water under different land‐use systems representative of the highlands of northern Thailand. In addition, we determined the distribution of organic C (OC) and N (ON) between two operationally defined fractions of DOM. Samples were collected in small water catchments including a cultivated cabbage field, a pine plantation, a secondary forest, and a primary forest. The mean concentrations of DOC and DON in bulk precipitation were 1.7 ± 0.2 and 0.2 ± 0.1 mg L^–1, respectively, dominated by the hydrophilic fraction. The throughfall of the three forest sites became enriched up to three times in DOC in the hydrophobic fraction, but not in DON. Maximum concentrations of DOC and DON (7.9–13.9 mg C L^–1 and 0.9–1.2 mg N L^–1, respectively) were found in samples from lysimeters at 5 cm soil depth. Hydrophobic OC and hydrophilic ON compounds were released from the O layer and the upper mineral soil. Concentrations of OC and ON in mineral‐soil solutions under the cabbage cultivation were elevated when compared with those under the forests. Similar to most temperate soils, the concentrations in the soil solution decreased with soil depth. The reduction of OC with depth was mainly due to the decrease of hydrophobic compounds. The changes in OC indicated the release of hydrophobic compounds poor in N in the forest canopy and the organic layers. These substances were removed from solution during passage through the mineral soil. In contrast, organic N related more to labile microbial‐derived hydrophilic compounds. At least at the cabbage‐cultivation site, mineralization seemed to contribute largely to the decrease of DOC and DON with depth, possibly because of increased microbial activity stimulated by the inorganic‐N fertilization. Similar concentrations and compositions of OC and ON in subsoils and streams draining the forested catchments suggest soil control on stream DOM. The contribution of DON to total dissolved N in those streams ranged between 50% and 73%, underscoring the importance of DOM for the leaching of nutrients from forested areas. In summary, OC and ON showed differences in their dynamics in forest as well as in agricultural ecosystems. This was mainly due to the differing distribution of OC and ON between the more immobile hydrophobic and the more easily degradable hydrophilic fraction.