Nutrient Release Rates Research Articles

Three-dimensional numerical simulation of water quality and sediment-associated processes with application to a Mississippi Delta lake

A three-dimensional water quality model was developed for simulating temporal and spatial variations of phytoplankton, nutrients, and dissolved oxygen in freshwater bodies. Effects of suspended and bed sediment on the water quality processes were simulated. A formula was generated from field measurements to calculate the light attenuation coefficient by considering the effects of suspended sediment and chlorophyll. The processes of adsorption–desorption of nutrients by sediment were described using the Langmuir Equation. The release rates of nutrients from the bed were calculated based on the concentration gradient across the water–sediment interface and other variables including pH, temperature and dissolved oxygen concentration. The model was calibrated and validated by applying it to simulate the concentrations of chlorophyll and nutrients in a natural oxbow lake in Mississippi Delta. The simulated time series of phytoplankton (as chlorophyll) and nutrient concentrations were generally in agreement with field observations. Sensitivity analyses were conducted to demonstrate the impacts of varying suspended sediment concentration on lake chlorophyll levels.

Accelerated testing of nutrient release rates from fertiliser granules

A method is developed for estimation of accelerated nutrient release rates into a soil from particulate fertiliser formulations, by carrying out release studies at elevated temperatures. This can provide time-saving targetting of samples to be subsequently used in field trials. The rate of total phosphorus leaching into water from a standard commercial fertiliser product embedded in grey, deep, sandy duplex soil was monitored using inductively coupled plasma-optical emission spectroscopy. The time dependence of phosphate concentration at a given temperature was found to obey simple first-order kinetics. These data fitted an Arrhenius relation within acceptable limits, enabling release rates at ambient field temperatures to be estimated significantly faster than by field experiment. A release rate calculated for the accelerated test, i.e. (1.2 ± 0.1) × 10−3/min, when compared with a more standard-type pot-trial leaching rate for the corresponding temperature of 2.3 × 10−4/min, gives a direct correlation of ~1 : 1 when corrected for concentration differences.

Seasonal and Spatial Variations of Benthic Impacts of Mussel Longline Farming in a Eutrophic Danish Fjord, Limfjorden

Limfjorden has a long tradition of mussel production based on wild stock fisheries. As a result of environmental concern of adverse effects of fishery, farming of mussels on off-bottom longlines has been well established during the past 5 y. Limfjorden is heavily eutrophic, resulting from intensive agricultural production in the catchment area of the estuary. Stimulation of sedimentation and subsequent benthic impacts of mussel farms were investigated with a focus on seasonal and spatial variations. The seasonal study was performed along a transect from inside one of the mussel farms to a reference station 75 m northeast of the farm every season during 1 y (summer to summer). The effects depended significantly on the variation in hanging mussel biomass along the transect as the effects on benthic total oxygen uptake, nutrient release, and sulfate reduction rate was at its maximum at the station with the largest hanging biomass. In addition, stimulation of the mussel farm was strongest during the summer when NH4 and PO43- release was increased by 400% and 740%, respectively, compared with the reference station. The concentration of reduced sulfur was only significantly stimulated (58%) by the mussel farm during the second summer season, which was characterized by total oxygen depletion in the bottom water and linked to the unavailable oxygen for reoxidation. A significant spatial variation in the benthic impacts was also observed. Eight differently situated mussel farms with variation in water depth and mussel biomass on the longlines were examined. The stimulation on total sedimentation rates, total oxygen uptake (TOU), and NH4 and PO43- flux of the mussel farms was in the range of 0–92%, 0–67%, 52–290%, and 0–700%, respectively. Furthermore, stimulation of TOU and NH4 flux was significantly linked by linear regression to the sedimentation rate of particulate organic carbon and particulate organic nitrogen, respectively. Hence, this study shows strong evidence that mussel farms stimulate the benthic degradation rates in an area that is already highly affected by other sources of organic input.

Decomposition of Nothofagus wood in vitro and nutrient mobilization by fungi

The role of fungi in determining rates of wood decomposition and nutrient release in forest ecosystems is poorly understood. The decomposition of wood from three species of Nothofagus by 12 species of widely occurring New Zealand wood-decay fungi was investigated in vitro under standardized conditions. Wood mass loss varied strongly among fungal species and to a lesser extent with the species of wood. The species of fungi in this study were divided into three groups based on the extent of mass loss after 15 weeks: (1) rapid (>65% reduction in mass, Fomes hemitephrus , Pleurotus purpureoolivaceus , Trametes versicolor , and Ganoderma cf. applanatum), (2) intermediate (15%–30%, Phellinus sp., Schizopora radula , Phellinus nothofagi , and Skeletocutis stramenticus ), and (3) slow (<10%, Armillaria novaezelandiae , Postia pelliculosa , Australoporus tasmanicus , and Laetiporus portentosus ). For several fungal species, the final contents of nitrogen, phosphorus, and calcium in the remaining wood exceeded the initial nutrient contents in the wood, indicating that nutrient sequestration from the supporting soil matrix occurred during decomposition. Nutrient dynamics during decomposition varied with wood species, but the variation among different fungal species was much greater, indicating that fungal diversity is an important factor in determining nutrient flux in decaying wood.

Effects of sunlight on the production of dissolved organic and inorganic nutrients from resuspended sediments

A series of laboratory-based and field experiments was conducted to address the effects of sunlight-exposed resuspended sediments on dissolved nutrient fluxes in two different water bodies. In suspensions of tidal creek sediments in 0.2 μm-filtered creek water, measurable increases in dissolved nutrients occurred after only 2 h of exposure to simulated sunlight. During a 6-h irradiation, nutrient release rates for total dissolved nitrogen (TDN) and phosphate were 2.2 ± 0.5 (standard error; S.E.) μmol g−1 h−1 and 0.09 ± 0.005 μmol g−1 h−1 (S.E.), compared to no significant changes in dark controls. The majority of nitrogen was released as dissolved organic nitrogen (87% on average) with lesser amounts of ammonium (13%). Continental shelf sediments resuspended in unfiltered seawater also released phosphate and TDN when exposed to sunlight, suggesting that this process can occur in a variety of marine and estuarine environments. The source material for inorganic nutrients appears to be associated with sediments rather than dissolved organic matter, as no significant changes in nutrient concentrations occurred in experiments with 0.2 μm-filtered creek water or seawater alone. Results suggest that photoproduction of dissolved nutrients from resuspended sediments could be an episodically significant and previously unrecognized source of dissolved organic and inorganic nutrients to coastal ecosystems. This process may be especially important for continental margins where episodic resuspension events occur, as well as in regions experiencing high riverine sediment fluxes resulting from erosion associated with deforestation and desertification.

Snail herbivory on submerged macrophytes and nutrient release: Implications for macrophyte management

Radix swinhoei (H. Adams) is a freshwater snail commonly found in shallow regions of Lake Taihu. This research estimated, based on experiments, the consumption rates of R. swinhoei on three young submerged plants ( Vallisneria spiralis, Hydrilla verticillata and Potamogeton malaianus) and its rates of nutrient release. Results showed that the snails consumed V. spiralis at the highest rate (23.34 mg g −1 d −1), P. malaianus at a lower rate (11.97 mg g −1 d −1), and H. verticillata at the lowest rate (7.04 mg g −1 d −1). The consumption rates on V. spiralis varied significantly, with snail size, ranging from 13.63 mg g −1 d −1 for large-size snails to 143.42 mg g −1 d −1 for small-size ones. The average nutrient release rates of snails grazing on different macrophytes were 45.93 μg PO 4-P and 0.58 mg NH 4-N g −1 d −1. The food species had a significant effect on NH 4-N release rates but not on PO 4-P. However, the snail size had a significant effect on PO 4-P release rates and not on NH 4-N. The present study indicates that through selective grazing and nutrient release, snails may impose a significant impact on the macrophyte community, which should be considered in managing the macrophytes of a lake.

Decomposition and nutrient release patterns of Pueraria phaseoloides, Flemingia macrophylla and Chromolaena odorata leaf residues in tropical land use systems

The study determined the decomposition and nutrient release patterns of Pueraria phaseoloides, Flemingia macrophylla and Chromolaena odorata leaf residues in young secondary forest and bush land use systems at Mfou, southern Cameroon. Residue mass loss after incubating fresh leaves for 120 days was C. odorata > P. phaseoloides > F. macrophylla. Mean weekly loss rates were 4.5, 3.9 and 2.8 per cent for C. odorata, P. phaseoloides and F. macrophylla , respectively. Across residue types, leaf mass loss was greater in the young secondary forest than in the bush land use. Soil texture and soil chemical properties were not correlated to leaf mass loss. Except for N, releases of P, K, Ca and Mg were of the order C. odorata ³ P. phaseoloides > F. macrophylla. Proportions of K released from the leaf residues were greater than those for the other nutrients. Half of the K was released within the first 7 weeks after incubation for C. odorata and P. phaseoloides. Among the land uses, release of K in the forest was 1.8 times faster than in the bush. The slowest releases, irrespective of type of leaf residue, were in Ca and Mg. The study concluded that among the planted fallows, Pueraria phaseoloides had the greatest rate of nutrient release. It could, therefore, ensure greater nutrient availability to associated crops. The leaves of C. odorata are rich in all nutrients; hence, short-season crops could benefit from this source of nutrients if the weed is well managed.

Effect of soy residue (okara) on in vitro protein digestibility and oil release in high-calorie emulsion stabilized by heated mixed proteins

Abstract This study investigated the influences of a dietary fiber source – namely okara powder (OKP) and pectinase-hydrolyzed okara powder (PHO) – on the physicochemical characteristics of a high-calorie emulsion (1 kcal/mL) stabilized by sodium caseinate and soy protein isolate at a ratio of 0.7:0.3, with caloric distribution from carbohydrate:protein:lipid of 55:15:30. The viscosity and microstructure of the emulsion containing 0.08% (w/v) crude fiber and its digesta were monitored during in vitro protein digestion by pepsin and trypsin, along with the interfacial protein content in the cream phase, the soluble protein content in the serum phase, and oil release during the course of digestion. Use of OKP induced the formation of a thick paste under acidic conditions, and oil could be released after 15 min digestion by pepsin. On the other hand, the presence of PHO increased the viscosity of the emulsion under acidic pH to less extent than did the OKP and delayed the release of oil for up to 105 min during peptic digestion. The oil could be released from PHO-containing digesta after tryptic digestion, both in the absence and presence of bile acid. The study indicated that the rate of nutrient release can be controlled by the presence and type of dietary fiber source.

Soil Physico-Chemical Properties and Crop Yield Improvement Following Lantana Mulching and Reduced Tillage in Rainfed Croplands in the Indian Himalayan Mountains

In the rainfed crop fields of the Central Himalayan Mountains in India, low soil moisture and low soil fertility are the two major constraints on crop yield. Therefore, an experiment was conducted to test the hypothesis of whether mixing a high-quality (high foliar N, P, and K and low lignin) organic residue (Lantana camara leaves) with the relatively low-quality leaf litter of oak (Quercus leucotrichophora) and pine (Pinus roxburghii) forests, and then mulching the organic residue, would enhance the rate of organic residue decomposition and nutrient release to contribute to the soil fertility and crop yield of wheat and paddy in rainfed crop fields. To achieve the study objectives, Lantana residue was mixed with oak and pine leaf litter in four different proportions and applied under three tillage frequencies in 36 experimental plots in a completely randomized block design. These four mulch combinations were also studied for dry matter decomposition and nutrient release pattern. Results showed that both the decomposition rate (k = 1.86 yr−1) and rate of nutrient release from decomposing litter (N = 0.208 mg d −1 and P = 0.042 mg d −1) for Lantana residue were much higher than the other litter combinations. Mixing Lantana with other mulch materials positively influenced the decomposition and nutrient release of otherwise slow decomposing oak and pine leaf litter. The 100% Lantana mulched plots recorded substantially higher soil moisture among the four mulching treatments. The 100% Lantana mulched plots also recorded significantly higher soil nutrients (NO3 +-N and PO4 −-P under wheat and NH4 +-N under rice crops) and rate of N-mineralization, and produced significantly higher wheat grain (920 to 1309 kg ha−1) and wheat and rice straw yield than the conventional practice of crop cultivation. Lantana residue thus proved to be a potential mulch to achieve soil moisture conservation, soil fertility enhancement, and higher crop yields in the region.

Numerical simulation of sediment‐associated water quality processes for a Mississippi delta lake

AbstractThree major sediment‐associated processes were presented to describe the effects of sediment on the water quality processes, including the effect of sediment on the light intensity for the growth of phytoplankton (PHYTO), the adsorption–desorption of nutrients by sediment and the release of nutrients from the bed sediment layer. A formula was generated from field measurements to calculate the light attenuation coefficient by considering the effects of concentrations of chlorophyll and suspended sediment (SS). The concentrations of adsorbed and dissolved nutrients because of adsorption–desorption were calculated using two formulas that were derived based on the Langmuir Equation. The release rates of nutrients from the bed sediment were calculated by considering the effects of the concentration gradient across the water‐sediment interface, pH, temperature and dissolved oxygen (DO) concentration. Model algorithms describing the adsorption and desorption of nutrients from sediment particles as well as the release of nutrients from bed sediment were tested using experimental data. These sediment‐associated water quality processes were included in a three‐dimensional (3D) water quality model, CCHE3D_WQ, developed by the National Center for Computational Hydroscience and Engineering (NCCHE), to simulate the concentrations of PHYTO and nutrients in a shallow Mississippi Delta lake with special emphasis on sediment‐related processes. The simulated concentration of PHYTO (as chlorophyll) and nutrients were generally in good agreement with field observations. This study shows that there are strong interactions between sediment‐associated processes and water quality constituents. Copyright © 2009 John Wiley & Sons, Ltd.

Effects of diet on release of dissolved organic and inorganic nutrients by the copepod Acartia tonsa

Acartia tonsa copepods are not limited to herbivory and can derive up to half their daily ration from predation on heterotrophic ciliates and dinoflagellates. The effects of an omnivorous diet on nutrient regeneration, however, remain unknown. In this study, we fed A. tonsa an exclusively carnivorous diet of either (1a) heterotrophic dinoflagellate Oxyrrhis marina or (1b) Gyrodinium dom- inans, (2) an exclusively herbivorous diet of Thalassiosira weissflogii diatoms, or (3) a mixed omnivo- rous diet. We measured the release rate, composition, and stoichiometry of dissolved organic carbon (DOC), dissolved organic phosphorus (DOP), and nitrogen (urea) in addition to the inorganic nutri- ents, ammonium (NH4 + ) and phosphate (PO4 3- ). Despite similar ingestion rates among treatments, as well as similar C:N ratios of food items, A. tonsa release rates of DOC and NH4 + were highest while feeding on a carnivorous diet and lowest while feeding omnivorously. In contrast, urea, on average, was a higher portion of total nitrogen released in the mixed diet treatment (32 to 59%). DOP release rates were only detectable in diets containing microzooplankton prey. Our results suggest that cope- pod diet plays an important role in determining the quantity and composition of regenerated C, N, and P available to phytoplankton and bacteria. Additionally, the uncoupling of ingestion and nutrient release rates and the variability in released ratios of dissolved C:N:P in our study suggests that stoi- chiometric models based exclusively on predator and prey C:N and N:P ratios may not be adequate in determining stoichiometry of total nutrient release.

Sediment Oxygen Demand and Nutrient Fluxes for a Tropical Reservoir in Singapore

Sediment oxygen demand (SOD) and nutrient flux studies were conducted for a tropical reservoir in Singapore in order to determine the approximate SOD and nutrient release rates from the sediments. SOD values obtained from laboratory experiments ranged from 1.4 to 3.3 g O2 / m2 -day . Similar results were also obtained by calculating SOD values from in situ DO measurements taken in the field. The nutrient flux study was performed in the laboratory at a constant temperature of 25°C in oxic and anoxic columns. Except for nitrate+nitrite , higher nutrient release rates were generally observed under anoxic conditions. The ammonium release rate was 0.06 g O2 / m2 -day under oxic conditions and 0.117 g O2 / m2 -day under anoxic conditions. The nitrate flux rate was 0.17 g O2 / m2 -day under oxic conditions but was negligible under anoxic conditions. Orthophosphate flux results were negative throughout the oxic incubation implying that sediments acted as a sink. The release rate of orthophosphate was 0.007 6 ...

Decomposition of organic amendment and nutrient release under the zai technique in the Sahel

In the West African Sahel, farmers use the zai technique to reclaim degraded cropland. Although the nutrients released by the decomposition of the amendments are central to the success of the technique, little is known regarding the impact of the zai pits on the decomposition process and whether the nutrient release is synchronized with plant requirements. The decomposition of millet stalks and cattle manure applied in zai pits or at the soil surface was studied in Niger using litterbags, under controlled irrigation on-station in 1999 and on-farm in 1999 and 2000 at two locations (Damari and Kakassi) with contrasting soils. In addition, a satellite trial was conducted in 2000 on-farm at the same locations to study the relative contribution of termites to manure decomposition. Only at Damari did termite presence enhance manure decomposition, by a factor three for surface placement compared to the zai pits. At Damari, zai pits enhanced the decomposition when termite activity was suppressed. Whereas manure decomposition proceeded two to three times faster than that of millet stalks at Damari, the type of amendment had no effect on decomposition rate at Kakassi. Nutrient release followed the trend of organic amendment decomposition except for K. When applied prior to the rainy season, nutrient release rate of organic amendments strongly exceeded plant nutrient uptake, which could lead to important leaching losses during the first 4–6 weeks after sowing, especially for N and to a lesser extent for K. However, at harvest, total nutrient absorption by plants was generally higher than the total amount released. The results indicate a highly site-specific response of amendment decomposition to zai and the need for a better timing of amendment application to reduce potential leaching losses, possibly through a split application.

Hazardous animal waste carcasses transformation into slow release fertilizers

Because of the need to address disposal of materials infected with pathogens new regulations have come into effect for the transport and disposal of dead farm animals or carcasses. For precautionary reasons, disposal to landfill, composting, biogas generation or fertilizer use are banned recycling paths because of incomplete knowledge about contamination transmission paths. Thermal treatment is recognized as a safe elimination process. Animal wastes have a high calorific value (above 16 MJ/kg). However, combustion of the organics leaves mineral residues (near 30%). The ashes contain mostly calcium and phosphate with some sodium, potassium and magnesium. We have examined the transformation of the ashes into a slow release fertilizer. We used a mixture of acids to partly dissolve the combustion residues and form slurry. In a second step, base was added to neutralize and solidify the reaction mixture. The final product was a whitish polycrystalline solid. Leaching tests were made to evaluate the nutrient release rate in laboratory columns. Water leachates were analyzed for up to ten pore-bed volumes and showed, as expected, large differences in release rates. Nitrate release was slowed and phosphate did not level even after ten pore-bed volumes. This demonstrates that insoluble precipitates (gypsum) contribute to control soluble ion release.

General empirical models for predicting the release of nutrients by fish, with a comparison between detritivores and non‐detritivores

Summary1. We derived models of nutrient release [nitrogen (N) and phosphorus (P)] by fish based on studies that directly measured the release rates from 56 species across a broad range of fish mass, feeding histories and temperature.2. We developed four separate models of nutrient release from multiple regression analysis: detritivore release rates of N and P, and non‐detritivore release rates of N and P.3. Fish mass explained most of the variance (78–92%) in release rates.4. Our predicted rates of release of P by fish (g ha−1 day−1) were similar to observed rates in the literature from other lakes.5. The influence of a shift in diet (planktivory to detritivory) by a single species (gizzard shad, Dorosoma cepedianum, a facultative detritivore) on nutrient release rates was estimated. During periods of detritivory, gizzard shad accounted for on average 39% (<1–96%) of all nutrients released by the fish assemblage, and increased total fish assemblage release rates on average by 59% (<1–331%) compared to when gizzard shad were modelled as planktivores.6. These models provide a rapid means for predicting the release of nutrients by fish assemblages and may facilitate more comprehensive comparisons of nutrient cycling by fish with other internal pathways.

Effects of periphyton stoichiometry on mayfly excretion rates and nutrient ratios

We evaluated the effects of periphyton elemental composition on the rate and ratios of N and P excretions by heptageniid mayfly larvae (Ephemeroptera). We predicted that excretion N:P ratios would relate primarily to mismatches between periphyton N:P and mayfly N:P. We immersed periphyton grown in a stream above (inlet: high N) and below (outlet: low N) a lake in P-amended or control (no P) water for 12 h to increase periphyton P content and reduce periphyton N:P. We then measured release rates and ratios of N and P from Cinygmula sp. larvae that had consumed periphyton from 1 of the 4 treatments. Reducing periphyton N:P increased consumer P excretion rates (range for P-amended periphyton: 0.055 ± 0.010 to 0.076 ± 0.022 μg P g−1 dry mass [DM] h−1; range for control periphyton: 0.027 ± 0.008 to 0.031 ± 0.006 μg P g−1 DM h−1). Mayflies that consumed P-amended periphyton from the inlet excreted N at a higher rate than did mayflies that consumed P-amended periphyton from the outlet (inlet P-amended: 0.661 ± 0.244 μg N g−1 DM h−1; outlet P-ammended: 0.245 ± 0.020 μg N g−1 DM h−1). We found some evidence for N:P imbalance between consumer and resource as a predictor of excretion N:P, but other factors also appear to influence nutrient release rates. Mayflies retained significantly more P than predicted by a stoichiometric mass-balance model, a result that suggests that mayflies might be able to store P when it available in their food and use it for growth when needed. Our model also suggests that assimilation efficiencies of N and P are more important determinants of excretion N:P than elemental imbalances between mayflies and their food. We conclude that elemental imbalances between lotic macroinvertebrates and their food resources play an important role in governing nutrient release rates and ratios, but that resource characteristics (e.g., periphyton community composition) and organismal physiology (e.g., nutrient assimilation efficiency) also must be considered. The possibly substantial role of these additional factors in determining nutrient excretion bears further investigation.

Nutrient Dynamics and Litter Decomposition in <i>Leucaena leucocephala</i> (Lam.) De Wit plantation in the Nigerian Derived Savanna

Nutrient contents and rate of litter decomposition were investigated in Leucaena leucocephala plantation in the University of Agriculture, Abeokuta, Ogun State, Nigeria. Litter bag technique was used to study the pattern and rate of litter decomposition and nutrient release of Leucaena leucocephala. Fifty grams of oven-dried leaf litter of the species was weighed into 0.2 mm mesh litter bag, 35 cm × 25 cm in size, these bags were closed at all ends. Eighteen litter bags were used for the studies. The litter bags were numbered and placed on the field (above ground) on 26th April 2005. Three bags were retrieved randomly from the field at 20-day intervals for 120 days. The nutrient concentration in L. leucocephala followed the trend N > P > Mg > Ca > K > Na for leaf litter and seed components. Nutrient concentration in twigs and pods ranked N > Mg > P > Ca > K > Na. Among all the litter components, leaf litter contributed more nutrients, especially nitrogen, than other litter components. High potassium concentration during the dry season was due to lack of rainfall to leach out the element. Lower magnesium content in leaf litter was due to chlorophyll decay. High N-flux reflected the quantity and quality of nitrogen in the soil. Mass loss was significantly correlated with calcium (P < 0.05). At 120 days, 80% of the litter had decomposed; this implies that decomposition rate was at 0.6% per day. A net immobilization of all the nutrient elements at 20 days was due to lack of rainfall to leach out the nutrients. High rate of decomposition at the early stage was due to less moisture content in the soil and high temperature. Litter decomposed more during the wet season than the dry season.West African Journal of Applied Ecology Vol. 13 2008: pp. 96-103

Effects of oxygen and nitrate on nutrient release from profundal sediments of a large, oligo-mesotrophic reservoir, Lake Mathews, California

Lake Mathews is a large, oligo-mesotrophic reservoir located in Southern California. The reservoir has elevated levels of nitrate and periodically experiences hypolimnetic anoxia. Experimental sediment-water chamber incubations and reservoir water quality monitoring were conducted to evaluate how oxygen and nitrate in overlaying water affect nutrient release from profundal sediments and internal nutrient loading. In experimental incubations, under nitrate-free anoxic conditions, sediment nutrient release rates were 3.4 ± 0.8 mg-P/m2 ·d for phosphate and 2.8 ± 1.2 mg-N/m2 ·d for ammonia (average ± standard deviation; n = 6). Oxygen repressed phosphate release and greatly diminished ammonia release from sediments in experimental incubations while nitrate only repressed phosphate release. Similar nutrient release dynamics were observed in the reservoir. Nutrient release rates estimated from seasonal nutrient profiles collected from the reservoir were 3.4 mg-P/m2 ·d for phosphate and 2.5 mg-N/m2 ·d for ammonia. Ammonia accumulation in the hypolimnion commenced with the onset of anoxic conditions, but phosphate accumulation did not start until nitrate disappeared from bottom waters approximately 6 weeks later. The time lag decreased total internal phosphorus loading by approximately 25% relative to hypothetical nitrate-free conditions. Laboratory and field data show that both oxygen and nitrate repress sediment phosphate release, likely via the maintenance of an oxidized surficial sediment layer that retains phosphate in iron-oxide complexes. However, only oxygen and not nitrate was effective in decreasing sediment ammonia release, likely by enhancing biological nitrification and assimilation in surficial sediments under oxic conditions. A number of in-lake management strategies have been developed to inhibit internal nutrient loading including calcium nitrate addition, aluminum sulfate addition, and oxygenation. In our view, the deliberate addition of nitrate to lakes and reservoirs poses several risks that must be carefully considered when evaluating strategies to control sediment phosphorus release.

Nutrient Release from Weathering of Purplish Rocks in the Sichuan Basin, China

Purplish soils having high fertility with mineral nutrients inherited from the parent rock are widely distributed in the hills along the Yangtze River, especially in the Sichuan Basin. Pot and field weathering experiments were conducted to mimic rock weathering and nutrient release processes in order to better understand soil fertility and nutrient compensation. Three types of purplish rock formations formed in the Jurassic period, Shaximiao (J 2s), Suining (J 3s), and Penglaizhen (J 3p), as well as one type formed in the Cretaceous period, the Chengqiangyan group (K 1c), were used in this study. Results showed that the soil formation rate was in the range from 11.2 to 19.6 mm every year, and rock weathering was in the order of J 3s > J 3p > J 2s > K 1c. Because more rock surface was exposed to sunlight and rainfall in field conditions, pot weathering was slower than field weathering. Nutrient release rates increased with rock weathering and was in the order similar to that of rock weathering: J 3p > J 3s > J 2s > K 1c. Potassium release was the most important in all rocks; after 2 years of weathering, 19.4% to 46.9% of K was released from the initial parent rocks, which suggested that K release from weathering could meet most of the crop K requirement in purplish soils. Thus, rapid release of nutrients from weathering of purplish rocks was key to nutrient replenishment and fertility of purplish soils.

Litter dynamics and soil properties under different tree species in a semi-arid region of Rajasthan, India

Litterfall and decomposition are the two main processes accounting for soil enrichment in agroforestry. The extent of enrichment in soil properties depends on the tree species, management practices and the quantity and quality of litter. A field investigation was carried out to study litterfall production, decay rates, release of nutrients and consequent changes in soil physicochemical properties under crowns of four multipurpose tree species (MPTs) in irrigated conditions in farm fields. The species were Prosopis cineraria (L.), Dalbergia sissoo (Roxb.) ex DC, Acacia nilotica (L.) Del. and Acacia leucophloea (Roxb.) Willd. Annual accretion of litter ranged from 36 to 54 kg tree−1 year−1 and was highest under D. sissoo and lowest under A. nilotica. Total litterfall production was in the order: P. cineraria > A. leucophloea > A. nilotica > D. sissoo. P. cineraria showed the highest NPK concentration in litter. For all MPTs, a large pulse of litterfall coincided with the winter season (November to February). Litter of P. cineraria decomposed fastest while that of A. nilotica was slowest. More than 95% of the leaf litter of P. cineraria decomposed in 6 months, of D. sissoo in 7 months and A. leucophloea and A. nilotica in 9 months. Decomposition rate of litter was highly correlated with neutral detergent fibre (NDF) (r = −0.94) and P (r = −0.91) concentration. N, P and K release were best correlated with NDF, acid detergent fibre (ADF), P, lignin, lignin/N and C/P ratios and NDF alone explained 88% to 94% of the variability in litter decomposition and nutrient release rates. There was significant build up of soil organic carbon and available NPK in the agrisilvicultural systems but also a decrease in soil pH. Build up in soil fertility was significantly correlated with litterfall and soil improvement was greatest under P. cineraria.