Organic Phosphorus Fractions Research Articles

Phosphorus Forms and Availability Assessed by 31P‐NMR in Successively Cropped Soil

Soils under no‐tillage gradually increase in organic matter and phosphorus (P) content from the top layer. Because of lack of knowledge about the organic phosphorus fraction contribution to plant nutrition, this research was conducted to estimate the availability of phosphorus fractions to plants. Soil samples of a very clayey Rhodic Hapludox that received 0, 156 and 312 kg P ha−1 were submitted to 15 successive crops in pots without replacing P extracted by plants. 31P nuclear magnetic resonance analysis was performed to detect P fractions before cultivation and after the sixth, ninth and fifteenth crops. Inorganic phosphorus was the unique P fraction acting as P source to plants in soils with previous P addition. Contribution of organic P was observed only when inorganic P content was extremely low, with plants showing severe P stress. Contribution of organic P was not enough to supply the required P for normal plant growth.

Effects of long-term fertilization on organic phosphorus fraction in paddy soil

The changes in the organic P fractions and microbial activity related to P transformation in the plow layer of paddy soil with long-term fertilization were investigated, in relation to the continuous application of chemical fertilizers (NPK), straw-based compost (Compost), and chemical fertilizers and compost (NPK + Compost) for 31 years. Continuous fertilizer application and rice cultivation led to a continuous decrease in ratio of organic P to total P in the plow layer with the lapse of years. In particular, the combined application of chemical fertilizers and compost accelerated the decrease in the organic P fractions, presumably due to the promotion of microbial activity in the plow layer, even though a high amount of organic P was inputted by compost. In this paddy soil, moderately labile organic P (MLOP) was the main organic P compound, which accounted for 70-90% of organic P, and showed the same tendency as that of the total organic P fraction during the test, irrespective of the treatments. Microbial P content and phosphatase activity, which were determined in the fresh soil collected in April 2003, were higher for the combination of chemical fertilizers and compost than for the single application of compost or chemical fertilizers, and showed a strong correlation with the amount of plant available P. In conclusion, the combined application of chemical fertilizers and compost could be an effective method to increase the plant availability of P in soils by promoting microbial activity.

Phosphorus mobilization in agroforestry: Organic anions, phosphatase activity and phosphorus fractions in the rhizosphere

In agroforestry systems on Ferralsols in the tropics, maize crop yields are low owing to a lack of P. However, some agroforestry tree species adapted to P-fixing soils may be able to utilize less available P sources and concurrently increase P availability to adjacent crops. Adaptations for enhanced P acquisition from P-fixing soils include exudation of low molecular weight (LMW) organic anions (OA) and phosphatase enzymes. We identified major organic anions, and measured acid phosphatase activity and P fractions in the rhizosphere soil of maize (Zea mays L.), as well as in the perennial species Grevillea robusta A. Cunn., Cassia spectabilis DC. (syn. Senna spectabilis (DC.) H.S. Irwin and R.C. Barneby), Tithonia diversifolia (Hemsl.) A. Gray, Eucalyptus grandis W. Hill ex Maiden and Cedrella serrata Royle. Maize and trees were grown simultaneously at field sites and in large pots. Rhizosphere soil of C. spectabilis contained at least 29 μmol oxalate g−1 soil, derived from a high exudation rate in the range of at least 5–10 μmol oxalate m−1 root day−1. Incubation of Ferralsols with much lower concentrations of citrate increased labile P fractions, but there was no clear relation between OA concentration and an increase in labile P fractions in rhizosphere soils, where P mobilization and P uptake occur at the same time. Acid phosphatase activity in rhizosphere soil of all species was two to five times greater in rhizosphere soil compared with bulk soil, and correlated in rhizosphere soil of G. robusta with a shift from organic P to inorganic P in soil P fractions. We conclude that organic anion exudation and acid phosphatase activity of tree roots may increase mobilization of P in the rhizosphere, the extent of which depends on the species, the organic anion and pH. However, it is unlikely that the extent of P mobilization will benefit adjacent crop plants unless crop roots exert insufficient P-mobilization effects themselves, and grow in the rhizosphere of tree roots.

Inorganic and organic phosphorus fractions in peat from drained mires in northern Finland

Soil samples from 15 eutrophic, 26 herb-rich, 15 tall-sedge, and 11 low-sedge drained peatland sites were analysed for easily soluble and aluminum, iron, and calcium bound phosphorus (P) using the Chang and Jackson sequential fractionation method. Compared to earlier investigations, where only total and easily soluble P contents (e.g. NH4OAc or dilute H2SO4 extractable P) in peat have been analysed, significantly higher differences between sites were observed. The eutrophic sites were characterized by four to six-fold greater Ca-bound organic P and two to three-fold greater Ca-bound inorganic P contents than on the other three site type groups, whereas the average Al-bound inorganic P content of the eutrophic sites was only one-third of that at the other site types. Substantial differences between sites were also observed for Fe-bound inorganic P, i.e. two to four-fold greater Fe-P contents were measured at the herb-rich sites compared with the other three site type groups. The stand volume growth in the 67 studied drained peatland sites correlated significantly with Al-bound organic P and Fe-bound inorganic and organic P. The study showed that a detailed fractionation and discrimination of different forms of soil P is important in increasing the understanding of the relationship between P availability and vegetation community types and stand growth on drained peatlands.

Frações de fósforo acumuladas em Latossolo argiloso pela aplicação de fosfato no sistema plantio direto

O aumento da disponibilidade de P na camada superficial do solo no sistema plantio direto pode resultar em maior transformação de P inorgânico em frações orgânicas de P de diferentes labilidades. Este trabalho teve como objetivo determinar as frações preferenciais de acumulação de P pela adição de doses de fosfato solúvel ao solo no sistema plantio direto. Após cinco anos e meio da instalação do experimento em um Latossolo Vermelho distroférrico típico argiloso, coletaram-se amostras da camada de 0-10 cm de tratamentos com diferentes doses acumuladas de fosfato solúvel: 0, 130, 180, 260, 360, 540 e 720, 980 e 1.240 kg ha-1 P2O5, aplicadas no inverno e, ou, no verão ao longo do tempo. Determinaram-se o P orgânico, inorgânico, total microbiano e seis frações inorgânicas e três orgânicas de P, em extração seqüencial com labilidade decrescente. As modificações observadas no P total, com a adição de fosfato ficaram restritas às frações inorgânicas. A fração moderadamente lábil foi o maior dreno do P adicionado. A contribuição das frações mais lábeis de P aumentou numa relação direta com as doses aplicadas. As frações orgânicas lábeis e moderadamente lábeis de P foram constituintes tão importantes como as respectivas frações inorgânicas, especialmente nas baixas adições de fosfato.

Accumulation of Soil Organic Phosphorus by Soil Tillage and Cropping Systems Under Subtropical Conditions

Changes in the concentration of organic phosphorus (P) fractions as affected by different soil tillage and cropping systems were analyzed in four long-term experiments established on two Oxisols (very clayey and clayey Rhodic Hapludox) and one Ultisol (sandy clay loam Rhodic Paleudult) of different clay contents, in southern Brazil. No tilled and conventional tilled soil under several crop sequences were collected in 1997 at three depths (0–2.5, 2.5–7.5, 7.5–17.5 cm) and analyzed for total, inorganic and organic soluble P by using different extractants (0.5 M sodium bicarbonate (NaHCO3) and 0.1 and 0.5 M sodium hydroxide (NaOH)). Microbial P was also determined. The sum of the organic P extracted by the three extractants and microbial P were considered the biological P pool, whereas the sum of inorganic and organic P in the residue was considered the geochemical P pool. Effects of soil tillage and cropping systems were mostly observed in the low activity clay soil (sandy clay loam Paleudult), with higher values under no-tillage and for soil cropped to oat+vetch/corn+cowpea rotation. Organic P accumulated mainly as the moderately labile P pool (P–NaOH). The geochemical P pool was higher than the biological P pool, and the biological reactions showed increasing importance in the low-active surface topsoil layer of no-tilled soils.

Labile Phosphorus in Soils of Forest Fallows and Primary Forest in the Bragantina Region, Brazil

ABSTRACTWe used the Hedley sequential extraction procedure to measure nine different organic inorganic soil phosphorus fractions in forest soil of the Bragantina region of Para, Brazil. We compared the labile fractions (resin‐extractable P + HCO3‐extractable inorganic and organic P) in Oxisols from three secondary forests (10, 20, and 40 years old) and a primary forest. These stands were located in an area that has supported shifting agriculture for approximately a century. After agricultural use, total P and labile P in soils of young secondary forests are diminished compared to the amounts presents in the primary forest soil. Within each stand, organic carbon content was a good predictor of labile organic and inorganic P, consistent with the large body of research indicating that mineralization of organic matter is important to plant nutrition in tropical ecosystems. During the reorganization of P pools during forest development, the pool of labile organic P (HCO3‐extractable) diminishes more than the other labile fractions, suggesting that it is directly or indirectly an important source of P for the regrowing forest vegetation. Across the four age classes of forest, the soil reservoir of labile P was equal to or greater than the total amount of P in the vegetation. If labile P measured by this method adequately represents P available to plants in the short term (as suggested by the current consensus), we would conclude that plant‐available P is reasonable abundant, and that the effects of agriculture on available P pools are detectable but not sufficient to compromise forest regrowth in this area.

Management of phytotoxicity of cobalt in tomato by chemical measures

Tomato ( Lycopersicon esculentum) cv. Alivikas was grown in refined sand at two levels of phosphorus (P), adequate (1.5 mM) and high (3.0 mM) each at excess (0.5 mM) cobalt (Co), supplied as cobalt sulphate. Apart from its own toxicity symptoms, excess Co induced iron (Fe) deficiency symptoms on young leaves after 6 days of metal supply. In addition excess Co decreased the biomass, concentration of Fe in different parts, chlorophyll a and b, Hill reaction activity and catalase activity and increased that of peroxidase, acid phosphatase, ribonuclease, Co concentration in various parts, carbohydrate and phosphorus fractions in leaves, more effectively at adequate than high P. High P had no harmful effects of its own on growth and metabolism of tomato. Withdrawal of excess Co after 12 days in tomato partially increased the biomass, concentration of proteins, chlorophyll a and b, Hill reaction activity and lowered the accumulation of sugars, starch, inorganic, organic phosphorus fractions and stimulated activity of acid phosphatase, peroxidase and ribonuclease more at adequate than high P. Addition of Fe along with excess Co ameliorated the effects of excess Co by partially increasing the biomass, concentration of chlorophyll a and b, iron, proteins, Hill reaction activity and catalase activity and elevated the accumulation of carbohydrates. Withdrawal of excess Co from tomato ameliorated the toxic effects of cobalt more effectively than application of iron (0.5 mM) as iron increased its own availability but failed to reduce the accumulation of Co, carbohydrate and phosphorus fractions.

Labile Phosphorus in Soils of Forest Fallows and Primary Forest in the Bragantina Region, Brazil1

Abstract We used the Hedley sequential extraction procedure to measure nine different organic and inorganic soil phosphorus fractions in forest soils of the Bragantina region of Para, Brazil. We compared the labile fractions (resin-extractable P + HCO3-extractable inorganic and organic P) in Oxisols from three secondary forests (10, 20, and 40 years old) and a primary forest. These stands were located in an area that has supported shifting agriculture for approximately a century. After agricultural use, total P and labile P in soils of young secondary forests are diminished compared to the amounts present in the primary forest soil. Within each stand, organic carbon content was a good predictor of labile organic and inorganic P, consistent with the large body of research indicating that mineralization of organic matter is important to plant nutrition in tropical ecosystems. During the reorganization of P pools during forest development, the pool of labile organic P (HCO3-extractable) diminishes more than ...

Effect of pine harvesting on leaf nutrient dynamics in young oak trees at NW Spain

The recovering of the native oak forest from pine plantations in unproductive areas is one of the scientific and politic challenges faced in NW Spain. We studied the effect of Pinus pinaster clearcutting on nutrient dynamics and availability for the late-successional species Quercus robur that dominates the understory of the pine canopy. Variations in leaf nutrient content (nitrogen, phosphorus, potassium and sodium) expressed on a leaf mass and area basis, were studied during a growing season, and changes in resorption efficiency, resorption proficiency, nutrient-use efficiency and nutrient-response efficiency were studied in oak leaves of harvested and undisturbed sites. Soil available nitrogen was 30% lower on the harvested sites, but no differences in the inorganic and organic soil phosphorus fractions were detected. Leaf specific mass was significantly higher in trees of the harvested sites, due to the lower leaf area found on these site. Leaf N concentration was 20% higher at the undisturbed sites, but pine harvesting did not modify the leaf P concentration. Consequently, nutrient content in plants seemed to reflect soil nutrient availability. The leaf N-to-P (between 18 and 20) ratio suggested that P was more limiting for growth than N for the young oaks. Pine harvesting increased the resorption rate and resorption efficiency of the young oaks for N, P and K. Pine harvesting decreased by 25% the minimum amount of N and K in the oak leaves (resorption proficiency), but no differences were found in the P resorption proficiency between sites. For P, the resorption proficiency values were nearer to those considered as complete resorption on both sites. Pine harvesting increased the leaf-level nutrient-use efficiency for all nutrients, but decreased the P uptake efficiency on the deforested sites. Our results suggested that changes in nutrient-use efficiency and nutrient resorption of young oaks after pine harvest may be influenced by changes in soil nutrient and light availability caused by pine harvest.

Influence of Anaerobic Pre-conditioning on Phosphorus Removal in Swine Manure by Aeration

A feasibility study with three pre-conditioning times was carried out to examine the effect of anaerobic pre-treatment on phosphorus removal by aeration. The results showed that soluble phosphorus could be decreased from 120 mg L −1 to about 40 mg L −1 within one-day aeration. However, soluble phosphorus concentration was found not affected by pre-conditioning, but more pH dependent. Anaerobic pre-treatment significantly increased the uptake of phosphorus by microbes indicated by the increase in concentration of organic phosphorus fraction. The presence of a large proportion of inorganic insoluble phosphorus considerably eclipsed the biological removal of soluble phosphorus, which suggested that the solids/liquid separation before aeration could be important to improve the biological uptake of soluble phosphorus.

Changes in Soil Phosphorus during 200 Years of Shifting Cultivation in Indonesia

We investigated the long-term effects of shifting cultivation on soil phosphorus pools to understand the effects of repeated intense disturbance on nutrient cycling in rain forests and to assess the sustainability of a widespread tropical agricultural system. We studied 24 sites that formed a gradient of cultivation history near a village in West Kalimantan, Indonesia, where long-fallow shifting cultivation has been practiced for at least 200 yr. The sites, including primary forest, had undergone 0–10 prior cycles (each ∼20 yr) of cultivation and forest regrowth. The number of swidden–fallow cycles experienced at a given site was determined through interviews. Using sequential chemical extraction of surface soils, we inferred changes in the distribution and availability of soil phosphorus during 200 yr of shifting cultivation. We examined the relative importance of fire vs. secondary forest biota in mediating the changes observed by heating soils in the laboratory to simulate the conditions of a slash fire and assessing data on root presence by depth. Over the first four cycles (∼80 yr), total phosphorus in the top 30 cm of soil increased by a mean of 73 kg·ha−1·cycle−1 (3.7 kg·ha−1·yr−1), much of it in the organic NaOH-extractable and the inorganic, concentrated HCl-extractable, fractions. Accounting for differences in original site fertility, total P did not change significantly over 2–10 cycles (∼200 yr), but the distribution shifted from both available and highly recalcitrant inorganic forms to available organic and occluded inorganic forms. NaOH-Pi decreased from 28% to 13%, and residual P decreased from 30% to 10% of total P. In contrast, NaOH-Po increased from 13% to 26%, and cHCl-Pi increased from 15% to 40%. The significant increase in soil organic phosphorus in combination with greater concentrations of fine roots deep in the soil profile suggest that deep-rooting fallow vegetation, rather than fire, is responsible for increasing, and maintaining, total phosphorus. The magnitude of changes induced by the fire are not great enough to account for the increases observed over several cycles, especially considering coincidental losses. Despite losses of nonoccluded Pi, we believe the availability of phosphorus may have been enhanced over 200 yr due to an increase in the stable organic phosphorus fraction. Long-fallow shifting cultivation, as practiced in West Kalimantan, can continue to support rural populations for some time.

Changes in soil organic matter and phosphorus fractions under planted fallows and a crop rotation system on a Colombian volcanic-ash soil

Planted tree or shrub fallows can help increase the fertility of degraded tropical soils. We investigated the effects of planted fallows of Indigofera (IND), Calliandra (CAL), and Tithonia (TTH); a natural, unmanaged fallow (NAT); and a maize/bean rotation (ROT) on the dynamics and partitioning of soil organic matter (SOM) and phosphorus (P). One year after treatment, samples were collected from a fine-textured volcanic-ash soil (Oxic Dystropept) of a mid-altitude hillside in southwestern Colombia. The SOM in the sand-size fraction (150–2000 μm) was subdivided into light (LL), intermediate (LM), and heavy (LH) fractions. Total soil P was also fractionated into inorganic (Pi) and organic (Po). Of the planted fallows, TTH most increased and NAT least increased plant-available Pi and Po. The amounts of C, N, and P in the LL and LM fractions of SOM followed the order, TTH>CAL>NAT>ROT>IND and CAL>TTH>IND>NAT>ROT, respectively. Total amounts of N, P, K, Ca, and Mg in the soil were significantly (P < 0.05) highest under TTH and lowest under NAT. The fallow and ROT systems did not affect the C/N, C/P, and N/P ratios in the soil but significantly did so in the LL and LM fractions of SOM. Significant correlations indicated that the P content in the LL and LM fractions of SOM may help determine the amounts of NaHCO3-extractable Pi and Po, which may therefore serve as sensitive indicators of `readily available' and `readily mineralizable' soil P pools, respectively, in the volcanic-ash soils of the Andes.

Some improvements of the fractionation method of organic phosphorus in calcareous soils

Some improvements of the fractionation of organic phosphorus (P 0) in calcareous soils were made on the basis of different soil extractants. Labile P 0 contained in soil microorganisms was extracted by fumigation of soil with chloroform (CHCl 3). The shaking time for extracting labile P 0, moderately labile P 0 and stable P 0 was shortened to 20 min, 2 h and 2 h, respectively, by using ultrasonic treatment. For extracting soil stable P 0 effectively, the concentration of NaOH was reexamined and 0.05 mol/l NaOH was proved to be suitable. According to the mineralization rate, stable P 0 can be subdivided into moderately stable P 0 and highly stable P 0 by adjusting the alkali solution to pH 3.00 as the suitable acidity.

Fractionation and characterization of epilimnetic soluble organic phosphorus in an Amazon lake

Fractionation and characterization of epilimnetic soluble organic phosphorus in an Amazon lake

The supply of phosphorus from native, inorganic phosphorus pools in continuously cultivated Mexican agroecosystems

Correlative and experimental approaches were used to determine the sources of phosphorus (P) that supply the labile P pools in nine soils from traditional Mexican agroecosystems. A correlation analysis was undertaken on phosphorus fractions, particle size fractions, soil pH and organic matter contents. Labile inorganic P (labile Pi) fractions correlated significantly with soil HCl-extractable Pi concentrations and soil pH. No significant correlations were found between labile phosphorus and any organic phosphorus pools or particle size fractions. Alfalfa ( Medicago sativa L.) was grown in small quantities of soil and then P fractionations were carried out on the alfalfa-depleted and original soils. Alfalfa consistently depleted labile inorganic phosphorus fractions in all soils, and in five of the nine soils, inorganic phosphorus associated with calcium phosphate minerals and/or Al or Fe hydrous oxides were significantly depleted under cultivation. The bicarbonate extracted organic phosphorus fraction was significantly depleted in one soil, but no other organic phosphorus fractions were significantly depleted by the alfalfa treatment. These results suggest that inorganic P fractions are critical sources of phosphorus in the traditional agroecosystems studied, and that soil calcium phosphate contents may in part determine where continually cultivated agriculture can be sustained without the use of synthetic fertilizers.

Waterlogging effects on organic phosphorus fractions in a toposequence of soils

Organic P plays a key role as a source and sink of P in hydromorphic soils. Our objective was to study the influence of hydromorphism on the distribution of soil organic P fractions. The research was performed at 4 sites from a toposequence of the Flooding Pampa (Argentina), which varied from a never-flooded upland site to a lowland site that remains waterlogged most of the year. All the soils were of the same age, parent material, vegetation type (native grassland), and land use. Total P content was higher in the soil of the lowland site than in that of the upland site. In all sites, at the 0–10 cm depth, most soil P was found in the organic fraction (ranging from 68 to 80%). The sum of the moderately resistant and resistant to mineralization fractions was greater than the labile and moderately labile fractions. The differences in the relative distribution of organic P fractions were minimal among sites. As a consequence, in these soils developed from the same parent material and with the same vegetation type (native grassland) and land use, the duration of waterlogging had little influence on the fractionation of soil organic P.

Effect of organic manure on organic phosphorus fractions in two paddy soils

We investigated the transformation of the organic P fractions from organic manure in two paddy soils (Ultisol, Entisol) and the influence of organic manure or cellulose on organic P under anaerobic conditions. The results obtained from the P fractionation experiment indicated that during the incubation labile and moderately labile organic P fractions increased in the Ultisol and decreased in the Entisol, which might be related to the difference in the organic matter content of both soils. Immediately after the application of organic manure, a large part of labile and moderately labile organic P supplied with the manure was transformed into moderately resistant organic P, possibly Ca- or Mg-inositol P were transformed into Fe-inositol P. During anaerobic incubation, the labile forms of organic P in the soils treated with organic manure were increased along with the incubation period in the first 4 weeks. The change in the moderately labile fraction was dramatic. It increased sharply in the first 2 weeks, then decreased, which was more pronounced in the soils treated with pig faeces. The moderately resistant fraction decreased during the whole incubation period. This indicated that under anaerobic conditions, the moderately resistant fraction can be transformed into labile and moderately labile organic P fractions, perhaps as Fe3+-inositol P is reduced to Fe+2-inositol P. Cellulose as an organic substrate had an increasing effect on organic P, especially when it was combined with inorganic P. Therefore, it is suggested that the application of inorganic P fertilizer combined with organic manure may be an effective way of protecting inorganic P against intensive sorption in soils.

Long-term crop rotation and fertilizer effects on phosphorus transformations: II. In a Luvisolic soil

The effects of different cropping systems, fertilizer, and lime on soil phosphorus (P) dynamics in soils developed under forest vegetation have received little attention. The objective of this study was to develop an understanding of P fractions and transformations in long-term rotation plots on a Luvisolic soil at Breton, Alberta. Results have shown that crop rotation and fertilizer application have affected more inorganic soil phosphorus (Pi) and organic phosphorus (Po) fractions, as determined by a sequential extraction procedure. Continuously cropped treatments, which had not received fertilizer, resulted in P drawdown of resin-extractable Pi (resin-Pi), sodium bicarbonate-extractable Pi (bicarb-Pi), sodium hydroxide-extractable Pi (NaOH-Pi), sodium bicarbonate-extractable Po (bicarb-Po), sodium hydroxide-extractable Po (NaOH-Po) and hydrochloric acid-extractable Pi (HCl-Pi) fractions. Only the residual-P fraction (insoluble Pi and stable Po forms) was unaffected. Addition of fertilizer had an effect on all P fractions except the NaOH-Po fraction. Phosphorus fertilizer treatments positively affected the Pi fractions and N fertilizer positively affected the bicarb-Po fraction. Lime application affected soil pH, which lowered NaOH-Pi levels and increased HCl-Pi levels through formation of more stable calcium phosphate compounds. Addition of lime also resulted in lower bicarb-Po levels. Cropping without using phosphate fertilizer has resulted in a 30–40% decline in total-P in the Breton plots in the Ap horizon. Continuous cropping, with a forage crop in the rotation, coupled with modest N and P fertilizer application, had the most positive effects on P cycling and transformations. Summerfallow had no apparent beneficial effects on P transformations. Key words: Soil P transformations, Luvisolic soil, P bioavailability, sequential extraction

A procedure for the simultaneous determination of total nitrogen and total phosphorus in freshwater samples using persulphate microwave digestion

In the absence of a suitable method for routine analysis of large numbers of natural river water samples for organic nitrogen and phosphorus fractions, a new simultaneous digestion technique was developed, based on a standard persulphate digestion procedure. This allows rapid analysis of river, lake and groundwater samples from a range of environments for total nitrogen and phosphorus. The method was evaluated using a range of organic nitrogen and phosphorus structures tested at low, mid and high range concentrations from 2 to 50 mg l −1 nitrogen and 0.2 to 10 mg l −1 phosphorus. Mean recoveries for nitrogen ranged from 94.5% (2 mg l −1) to 92.7% (50 mg l −1) and for phosphorus were 98.2% (0.2 mg l −1) to 100.2% (10 mg l −1). The method is precise in its ability to reproduce results from replicate digestions, and robust in its ability to handle a variety of natural water samples in the pH range 5–8.