Organic Pools Research Articles

Source of organic detritus and bivalve biomass influences nitrogen cycling and extracellular enzyme activity in estuary sediments

In aquatic ecosystems, natural processes that remove nitrogen from the biologically available pool (e.g. denitrification) have been intensively studied as an ecosystem function that reduces eutrophication. The quantity of sediment organic matter is a key driver of denitrification with percent organic content positively related to rates of nitrogen removal; however, few studies have investigated the influence of the quality of organic matter on nitrogen cycling in estuarine sediments despite shifts in primary producers with eutrophication. This laboratory study using intact benthic communities investigates the influence of various organic detritus sources, which vary in their C:N ratio, on nitrogen gas (N2) and solute fluxes and extracellular enzyme activity in estuarine sediments. A custom-built tank with a removable front plate was used with a planar optode film to image sediment oxygenation. Mangrove leaf detritus significantly increased the net N2 production in sediments, while the deposition of other detrital sources and control sediments produced net N2 consumption. Sulfatase activity was significantly reduced in the mangrove leaves and seagrass treatments, suggesting alteration of heterotrophic microbial activity with reducing oxygen conditions. Leucine aminopeptidase activity, indicating nitrogen cycling, was reduced in all treatments, suggesting the organic detritus provided a nitrogen supplement or reduced the activity of extracellular enzymes producing microbes. Bivalve biomass increased net nitrogen gas fluxes in some treatments. Our results indicate different detrital sources may have varying impacts on the removal of bioavailable nitrogen through denitrification and show that feedbacks in biogeochemical cycles may occur with changes in organic detrital source pools.

Combining microdialysis with metabolomics to characterize the in situ composition of dissolved organic compounds in boreal forest soil

Low-molecular weight organic compounds (LMWOC) play a key role in soil respiration. Thus, a detailed understanding of their dynamics is important for any attempt to describe carbon cycling of ecosystems. Measurements of LMWOC have been compromised by the perturbation associated with traditional soil extraction methods. Due to the fast turnover of LMWOC, there are potential losses of compounds caused by ex situ methods. These losses should be reduced by microdialysis, a novel soil sampling technique that samples the delivery of compounds through induced diffusive flux rates (DFR), representing what an active organism is exposed to in situ, in the undisturbed soil. The present study used microdialysis to measure the DFR of LMWOC in the O-horizon of a boreal forest soil in northern Sweden at four occasions during the middle to end of the growing season. LMWOC was analyzed by untargeted metabolomic profiling, which identified 50 compounds, and targeted mass spectrometry, which quantified seasonal shifts in the abundance of 36 amino acids, organic acids and sugars. The amino acid proportion of the DFR decreased by 85% during the growing season. However, because the decrease in amino acids was compensated for by increases in organic acid and sugar fluxes, the total DFR remained approximately constant during the season. The organic acid pool was dominated by lactate (80%) in the middle of the season and consisted mainly of oxalate (40%) during the later season. A lower amount and more diverse composition of sugars were found compared to previous studies. Six sugars and polyols were detected, with mannitol, arabitol and glucose being the most abundant. Microdialysis has previously been explored to monitor amino acids and inorganic compounds, but the present research demonstrates the potential of the in situ microdialysis sampling when combined with metabolomics. This broadens the application of microdialysis to describing quantitative and qualitative changes in the dynamic composition of the DFRs in undisturbed soil and a similar approach may be used in future studies to study the actual soil processes governing soil LMWOC.

Soil Microbial Biomass Size and Nitrogen Availability Regulate the Incorporation of Residue Carbon into Dissolved Organic Pool and Microbial Biomass

Microbial biomass (MB) plays a critical role in residue decomposition and soil organic matter (SOM) turnover. We investigated the effects of the initial size of soil MB pool and nitrogen (N) availability on the incorporation of ryegrass residue carbon (C) into microbial biomass C (MBC) and dissolved organic C (DOC). Soils from a row crop system (CS) and an adjacent grass sod (GS) were preincubated with (i.e., increased MB) and without (i.e., unchanged MB) glucose to produce soils with two levels of initial MB size before residue additions. Ammonium sulfate was added to test the effect of N availability. Residue addition increased DOC production in both soils, regardless of initial MB size and N availability, contributing 9 to ∼22% to the total pool. Residue quality was observed to affect the incorporation of residue C into DOC, which depended on soil type and initial MB size. However, the assimilation of residue C into MBC was not affected by the quality of ryegrass residue. Compared with the control (which did not have residue and N additions), a significant increase in SOM‐derived MBC by residue addition was observed in CS without glucose preincubation but not in the glucose preincubated CS and in GS. This indicated that the primed MBC by residue addition depended on initial MB size and soil type. The assimilation of residue C into MB was marginally inhibited by the preincubation with glucose in GS but was promoted in CS. With the addition of extra N, higher ryegrass‐derived MBC was observed in CS with glucose preincubation compared with the corresponding treatments without preincubation, which was not found in GS. These results suggested that residue‐derived MBC was not only regulated by initial MB size and N availability but also was affected by soil management history. However, N addition reduced ryegrass‐derived DOC production in GS without glucose preincubation. Apparently, both soil MB size and N availability largely affected the assimilation and incorporation of residue C in soil labile pools (i.e., DOC and MBC), and the extent of this relationship varied between two agricultural soils.Core IdeasRyegrass residue addition primed the production of SOM‐derived DOC and MBC.Residue quality affected ryegrass‐derived DOC in crop soils.N addition reduced ryegrass‐derived DOC in grass soils.N addition increased ryegrass‐derived MBC in crop soils.Initial MB and N availability regulated the incorporation of residue C into DOC and MBC.

Food Web Functions and Interactions During Spring and Summer in the Arctic Water Inflow Region: Investigated Through Inverse Modeling

We used inverse modelling to reconstruct major planktonic food web carbon flows in the Atlantic Water inflow, east and north of Svalbard during spring (18-25 May) and summer (9-13 August), 2014. The model was based on three intensively sampled stations during both periods, corresponding to early, peak, and decline phases of a Phaeocystis and diatom dominated bloom (May), and flagellates dominated post bloom stages (August). The food web carbon flows were driven by primary production (290 to 2850 mg C m-2 d-1), which was channeled through a network of planktonic compartments, and ultimately respired (180 to 1200 mg C m2 d-1), settled out of the euphotic zone as organic particles (145 to 530 mg C m-2 d-1), or accumulated in the water column in various organic pools. The accumulation of dissolved organic carbon was intense (1070 mg C m-2 d-1) during the early bloom stage, slowed down during the bloom peak (400 mg C m-2 d-1), and remained low during the rest of the season. The heterotrophic bacteria responded swiftly to the massive release of new DOC by high, but decreasing carbon assimilation rates (from 534 to 330 mg C m-2 d-1) in May. The net bacterial production was low during the early and peak bloom (26 to 31 mg C m-2 d-1) but increased in the late and post bloom phases (> 50 mg C m-2 d-1). The heterotrophic nanoflagellates did not respond predictably to the different bloom phases, with relatively modest carbon uptake, 30 to 170 mg C m2 d-1. In contrast, microzooplankton increased food intake from 160 to 380 mg C m2 d-1 during the buildup and decline phases, and highly variable carbon intake 46 to 624 mg C m2 d-1, during post bloom phases. Mesozooplankton had an initially high, but decreasing carbon uptake in May (220 to 48 mg C m-2 d-1), followed by highly variable carbon consumption during the post bloom stages (40 to 190 mg C m-2 d-1). Zooplankton shifted from herbivory (92-97% of total food intake) during the early bloom phase to a herbivorous, detritovorous and carnivorous mixed diet as the season progressed.

Ash application enhances decomposition of recalcitrant organic matter

Harvesting whole-tree biomass for biofuel combustion intensifies removal of nutrients from the ecosystem. This can be partly abated by applying ash from the combustion back to the system, as the ash is rich in nutrients. Ash is very alkaline and ash application raises soil pH, which in turn can stimulate microbial activity and thus decomposition and mineralization. Our aim was to test if ash induced decomposition activity was associated with enhanced turnover of recalcitrant, i.e. relatively old, organic pools. Two experiments were conducted in the same coniferous plantation after the application of 0, 3, 4.5 and 6 t ash ha−1, and 0, 3, 9, 15 and 30 t ash ha−1, respectively. We used natural abundance of 15N in mosses, mites and ectomycorrhizal fungi 26 months after ash application, as well as temporal variation in δ15N values of ectomycorrhizal fungi, as an indicator of decomposition of recalcitrant organic matter in the first experiment. Furthermore, in the second experiment we used measurements of extracellular manganese peroxidase activity almost 4 years after ash application as an indication of potential decomposition of lignin, an important component of recalcitrant organic matter.The δ15N signature increased significantly for ectomycorrhizal fungi, dead moss, Nothroid and Gamasida mites, and manganese peroxidase activity tended to increase, with increasing ash doses. This suggests that ash application stimulates turnover of recalcitrant organic matter, which can increase the available pool of nitrogen in the system. This will potentially enhance the fertilizer value of ash. However, the δ15N in ectomycorrhizal fungi tended to peak at 18 months after ash application, before decreasing, suggesting that the turnover of recalcitrant organic matter is reduced again with time.

Phosphorus distribution after three decades of different soil management and cover crops in subtropical region

The no-tillage system combining winter cover crops and crop rotation may increase the efficiency use of soil P and phosphate fertilizer. The objective of this study was to evaluate the effect of three decades of different soil management systems and winter cover crops on the fractions of P in a clayey Oxisol of Paraná State, Brazil. The bi-factorial experiment with three replicates was established in 1986. The main plots consisted of seven winter cover crops. In the subplots, two tillage systems were used: no-tillage and conventional tillage. The soil was sampled in the 0–10 cm soil layer in each plot in the post-harvest of the corn (March 2015), the flowering of the winter cover crops (September 2015) and at soybean flowering (February 2016). Samples from a native forest adjacent to the experimental area were also taken for comparison. The different pools of P were evaluated. Results indicated that after 30 years of no-till with phosphate fertilizers addition, the amount of P organic pools and type of P organic compound were very similar to those observed in the natural biome. The no-tillage guaranteed higher inorganic and organic P, both rapid and moderately available, in comparison to the conventional tillage system. However, in the conventional tillage (40 years − 80 plows + 160 harrows), despite the addition of 3 Mg ha−1 of P2O5, the available and moderately labile P content in soil surface were very similar to soil under natural forest. In the natural biome, the amount of P stored in soil microbial biomass was stable in the time showing a homeostatic equilibrium. Nevertheless, in cultivated soil and fertilized with inorganic phosphate, there was a synchronism between plant demand and storage in soil microbial biomass. Except for P stored in soil microbial biomass, the other P pools, regardless its nature (organic or inorganic) or degree of bioavailability, had despicable or absent effect of winter cover plants, compared to winter fallow.

Characterizing Organic Carbon Storage in Experimental Agricultural Ditch Systems in Northeast Arkansas

Core Ideas Agricultural drainage ditches can be considered as carbon sinks. Agricultural ditches can provide an optimum environment for carbon sequestration. Soil topography influences spatial carbon storage within wetlands. Soil bulk densities influence carbon storage in wetlands. Agricultural ditches are capable of many ecological functions, including flood control and edge of field nutrient filtration. This study investigated the potential for carbon sequestration within mowed and unmowed experimental conventional and controlled (with weirs) agricultural drainage ditches. The study analyzed and compared spatial and temporal variation in soil organic carbon (OC) concentration (g C kg–1) and OC pool (kg C m–2) within a 3‐cm soil depth between treatments. Soil OC concentrations were quantified through combustion of organic matter (OM) at 400°C in a muffle furnace for 16 h using the loss on ignition (LOI) method. Soil bulk density was also determined for each ditch treatment. In both summer and winter, mean soil C concentration in ditches with weirs was similar to mean soil C concentration in ditches with no weirs (16.68 ± 0.49 vs. 16.47 ± 0.46 g C kg–1 in summer; 14.47 ± 0.75 vs. 16.27 ± 0.72 g C kg–1 in winter). Similar bulk densities (0.67 Mg m–3, on average) and OC contents in ditches furnished comparable C pools in ditches with weirs and no weirs respectively (28.08 ± 0.75 vs. 27.88 ± 0.68 kg C m–2 in summer; 26.44 ± 1.56 vs. 30.24 ± 1.40 g C kg–1 in winter). The studied drainage ditches can therefore be considered for their contributions to the C sink, given the high values of C pool observed in the ditch treatments. This suggests agricultural drainage ditches can offer a suitable environment for C sequestration.

Nitrogen dynamics of decomposing Scots pine needle litter depends on colonizing fungal species.

In boreal ecosystems plant production is often limited by low availability of nitrogen. Nitrogen retention in below-ground organic pools plays an important role in restricting recirculation to plants and thereby hampers forest production. Saprotrophic fungi are commonly assigned to different decomposer strategies, but how these relate to nitrogen cycling remains to be understood. Decomposition of Scots pine needle litter was studied in axenic microcosms with the ligninolytic litter decomposing basidiomycete Gymnopus androsaceus or the stress tolerant ascomycete Chalara longipes. Changes in chemical composition were followed by 13C CP/MAS NMR spectroscopy and nitrogen dynamics was assessed by the addition of a 15N tracer. Decomposition by C. longipes resulted in nitrogen retention in non-hydrolysable organic matter, enriched in aromatic and alkylic compounds, whereas the ligninolytic G. androsaceus was able to access this pool, counteracting nitrogen retention. Our observations suggest that differences in decomposing strategies between fungal species play an important role in regulating nitrogen retention and release during litter decomposition, implying that fungal community composition may impact nitrogen cycling at the ecosystem level.

Grazing and enclosure alter the vertical distribution of organic nitrogen pools and bacterial communities in semiarid grassland soils

Different grazing management practices have a significant impact on the sustainability of grassland ecosystems. This study invested the vertical distribution of soil nitrogen (N) forms and soil microbial community structures in a semiarid grassland ecosystem under different grazing management practices in Inner Mongolia. Soil samples were collected from three semiarid grassland plots subjected to different long-term management practices namely, free grazing (FG) and two different periods of enclosure (E83, enclosed since 1983 and E97, enclosed since 1997). The soil organic nitrogen (N) pools were analyzed by classical methods, and the bacterial community was assessed by PCR-DGGE and high-throughput sequencing. The surface soil N-supplying capacity was in the order of E97 ≥ E83 ≥ FG. The soil ammonium N, amino N, and N-supplying capacity were greater in the enclosed plots than in the FG plot. Additionally, the 0–40-cm soil layer showed the influence of different management practices on the soil properties. The structure and diversity of the soil microbial community also varied with the management type. The soil organic N composition was significantly related to the soil bacterial community structure and microbial categories. An appropriate number of years of fencing helps to improve the soil surface nutrient status, whereas overgrazing and prolonged enclosure are not conducive to the restoration of soil nutrients. Different grazing management practices can affect the microbial community structure and turnover of soil N in grasslands.

Slow sand filtration for biofouling reduction in seawater desalination by reverse osmosis

Control of the organic substrate pool that determines the microbial growth potential (MGP) of feedwater in seawater reverse osmosis (SWRO) is a challenge unresolved in conventional or advanced membrane pretreatment. Slow sand filtration (SSF) combines filtration with biodegradation, but its capability of reducing MGP, proteins and carbohydrates on seawater feeds is not known. Two SSF, one constructed with new media (newSSF) and one from a previous filtration run (oldSSF), reduced MGP as measured in a growth assay with the marine organism Pseudoalteromonas songiae by one order of magnitude after maturation periods of 76 and 61 days, respectively. The reduction of the amount of biopolymers deposited on the surfaces of SWRO membranes in laminar fluid flow cells was significant with filtrates from biologically non-acclimated SSF (proteins: 60% (oldSSF) and −66% (new SSF), carbohydrates: 75% (oldSSF) and −70% (newSSF)) and an even greater reduction was observed after filter maturation (proteins: 81% (oldSSF) and −76% (new SSF), carbohydrates: 88% (oldSSF) and −88% (newSSF). Turbidity was less than 0.3 nephelometric turbidity units (NTU) and silt density index (SDI) < 4 immediately after startup and during the 181 days operating period regardless of the oscillations of the raw sea water quality. Filtration and biological activity were restricted to the top 30 cm of the media column, with no significant further contribution of the deeper media layers to filtrate quality.

Formation of CX3R-type disinfection by-products during the chlorination of protein: The effect of enzymolysis

It is commonly acknowledged that proteins, comprising a large portion of dissolved organic nitrogen pool, are important precursors of disinfection by-products (DBPs). Moreover, proteins can also undergo degradation (e.g. enzymolysis) in aquatic environment or during biological treatment processes. In this study, a well-characterized protein, bovine serum albumin (BSA) was used as model compound to investigate the effect of enzymolysis on the formation of four classes of CX3R-type DBPs (i.e. trihalomethanes, haloacetaldehydes, haloacetonitriles, and haloacetamides) during chlorination. After enzymolysis, the three-dimension structure of BSA was destroyed, increasing the accessibility of inside amino acids to chlorine, thus enzyme-hydrolysed BSA exhibited higher chlorine demand and higher formation of total organic halogen (TOX) than un-hydrolysed BSA. Generally, enzyme-hydrolysed BSA produced more CX3R-type DBPs, which is due to the facilitation of fragmentation of BSA by enzymolysis. In the presence of bromide, enzymolysis promoted the formation of TOX to a greater extent than in the absence of bromide. The effect of enzymolysis on pre-oxidation efficiency in the control of CX3R-type DBPs was also investigated. It was found that the reduction in CX3R-type DBP formation from chlorinated enzyme-hydrolysed BSA was more significant than that from chlorinated un-hydrolysed BSA, but the former still exhibited higher calculated cytotoxicity than the latter, indicating the removal of CX3R-type DBP precursors was indirectly offset by the effect of enzymolysis. This study provides insights into the effect of the structural transformation of protein during water treatment processes on CX3R-type DBP formation.

Transformation of organic phosphorus compounds during 1500 years of organic soil formation in Bavarian Alpine forests – A 31P NMR study

In Bavarian alpine forests, long-term input of organic matter from conifer species has built up thick organic surface layers, thus immobilized substantial soil organic phosphorus upon calcareous parent materials. The composition of these phosphorus stocks, however, remained unrevealed. Therefore, we analyzed organic phosphorus species in three so called Tangelhumus (Histosol according to World Reference Base) profiles with different radiocarbon ages using alkaline extraction followed by one-dimension and two-dimension 31P nuclear magnetic spectroscopy in order to understand changes in organic phosphorus composition with soil organic matter genesis. A diverse phosphorus pool composed of monoesters, diesters, and phosphonates was detected. However, in these Tangelhumus soils only monoesters accumulated overtime and eventually dominated the organic phosphorus pool after 1558 years. Diesters and phosphonates did not show any significant accumulation pattern. The origin of monoesters varied with soil depth and soil age: monoesters in young surface soils were actually hydrolysis products of diesters released by plant and microbial cells during alkaline extraction; whereas in older soils monoesters originated largely from Ca-bound precipitates formed after long-term in situ decomposition of organic matter and stabilization by Ca2+. Myo-inositol hexakisphosphate was below detection limit in all soils. The presence of its scyllo and other isomers likely pointed to direct synthesis by microorganisms instead of microbial epimerization from myo-inositol hexakisphosphate. In general, the overall picture of phosphorus speciation manifested as the continuous degradation of organic matter and accumulation of monoesters. Therefore, the current data indicated no involvement of humification process but favored the soil continuum model.

Phosphorus pool responses under different P inorganic fertilizers for a eucalyptus plantation in a loamy Oxisol

Phosphate fertilizers play an important role in plant nutrition. Different P fertilizer sources such as high-solubility (simple superphosphate, SSP), low-solubility (rock phosphate, RP) and complex superphosphate (CSP) are available for plant supplementation. The objective of this study was to investigate the short- and long-term redistribution of soil P after application of different P sources at establishment of an Eucalyptus forest stand. We carried out two experiments to identify the short- and long- term changes in a Brazilian Oxisol. To property identify the P pools in different times, Hadleýs fractionation methodology was applied to a long-term studies and citrate and oxalate to short-term. From zero to 180 days, the soluble P fractions were not altered in the non-fertilized treatment. Under SSP, a slight increase in this P fraction was found until 30 days, followed by a decrease in later evaluations. During the same period, a slight reduction in Pi extracted by citrate and oxalate was found under the control and a large reduction (approximately 50%) under the SSP treatment. Intermediate behavior was observed under the CSP and RP treatments, whereas there was an increase in P-citrate and P-oxalate until 30 days followed by a reduction afterwards. These results suggest that this pool comprises a potential bioavailability of P to plants. Different fertilizer sources did not increase the most recalcitrant P pool. However, different sources increased the organic P pool, mainly organic moderately labile P at long-term evaluations. Organic labile pools showed a fertilizer-specific response where CSP and RP increased respectively by 43% and 41% during the first year, and decreased to 39% in CSP treatment and 50% in RP treatment during the third year. Available P pool was highly dependent on inorganic and occluded pools and the organic pool acted predominantly as a sink of P on available and inorganic pools. The results reinforce the high level of recalcitrance of the organic pool and the fact that Eucalyptus plants must access pools of residual P in order to maintain their nutritional demands.

Combined effect of intercropping and minimum tillage on soil carbon sequestration and organic matter pools in the semiarid region of Brazil

This study aimed to evaluate the effect of two intercropping systems and minimum soil tillage in the semiarid region of Brazil on soil organic carbon (SOC) and pools of soil organic matter (SOM), compared with the native vegetation (NV). The first intercropping was cultivated with beans, sesame and pigeon pea, whereas the second was cultivated with cotton, maize, beans, sesame and pigeon pea. Two areas under NV, adjacent to the crop areas, were also sampled. Soil sampling were collected from 0–5, 5–10, 10–20, 20–30 and 30–50 cm layers in three plots per area to characterise the SOM (SOC, soil nitrogen, humic substances, microbial biomass, and mineralisable carbon). Our results demonstrated that, when compared with the NV, intercropping systems conducted with minimum soil tillage were effective in maintaining and sometimes increasing the levels and stocks of SOC and some SOM fractions such as microbial C and humic substances, and therefore, these systems can be an alternative form of sustainable soil management in the semiarid region of Brazil.

Pools of Silicon in Soils and their Contribution to Rice

Five major pools of silicon (Si) such as mobile Si, adsorbed Si, Si bound to organic matter, Si occluded in pedogenic oxides/hydroxides and amorphous Si were extracted from three soils namely one each from low, medium and high categories of plant available Si using a sequential extraction method. Different pools of Si extracted from soils were in the order of amorphous Si > occluded Si > organic Si > adsorbed Si and mobile Si. The mobile Si and adsorbed Si pools were the smallest pools of Si and ranged from 14.4 to 44.6 mg kg−1 and 4.9 to 89.4 mg kg−1, respectively. The content of organic Si and occluded Si ranged from 234 to 619 mg kg−1 and 476 to 1989 mg kg−1, respectively. Irrespective of the soils, amorphous Si was found to be the largest pool of Si ranging from 8019 to 16667 mg kg−1. Among different pools of Si, organic Si pool showed significant positive correlation with plant available Si while other pools had non-significant. A pot culture experiment was conducted using bulk soil samples with rice as test crop for a period of 60 days, and Si content and its uptake by rice was determined. Typic Rhodustalfs which is medium in plant available Si showed higher Si content (7.0%) and uptake (106 mg pot−1) compared to Chromic Haplusterts having high plant available Si. The correlation analysis to know the contribution of different Si pools to Si uptake by rice revealed that occluded, mobile, amorphous and adsorbed pools as the major contributors of Si to rice.

Earthworms and Phosphate‐Solubilizing Bacteria Stimulate Nitrogen Storage and Cycling in a Manured Arid Soil

Core Ideas Earthworm and PSB improved chemical properties of manure‐amended soils. Earthworm and PSB enhanced the increase of soil organic nitrogen pools by pig manure. Earthworm and PSB accelerated soil mineralization and nitrifier‐ denitrification. Improper reuse of manure could not only lead to low nitrogen‐use efficiency but also result in environmental pollution. Therefore, we studied the effect of manure amendment with or without organisms (earthworm and bacteria) on soil nitrogen cycling, aiming to develop a more scientific way to reuse manure. In this study, six treatments, including control (CK), manure (M), manure + slurry (MS), manure + slurry + earthworm (MSE), manure + slurry + bacteria (MSB), and manure + slurry + earthworm + bacteria (MSEB) were divided into two groups to study the effect of manure and the effect of organism on the soil nitrogen pools and nitrogen cycling, respectively. The results showed that manure amendment coupled with organisms increased total nitrogen from 0.538 ± 0.06 to 1.380 ± 0.22 g kg 1, and available nitrogen from 67.28 ± 5.22 to 113.11 ± 12.21 mg kg−1, and improved soil organic nitrogen pool by increasing acid‐hydrolyzable nitrogen (AHN) from 404 ± 39 to 974 ± 94 mg kg−1 and non‐acid‐hydrolyzable nitrogen (NAHN) from 133 ± 26 to 406 ± 155 mg kg−1. Based on the results of soil inorganic nitrogen and soil enzyme activities, we further found that earthworms and bacteria accelerated soil nitrogen cycling by stimulating soil mineralization (ammonification and nitrification) and nitrifier‐denitrification, providing more available nitrogen to plants. Thus, we conclude that manure coupled with organisms would be a more efficient way to reuse manure via increasing soil nitrogen storage and availability simultaneously.

Cultivation affected the level of inorganic phosphorus more than the organic pool of the Pampas soils in Argentina

ABSTRACTThe effects of land use on soil organic and inorganic phosphorus (P) stocks were assessed in the Pampas, Argentina. Three hundred and eighty-six paired sites widely distributed over an area of ca. 50 Mha were sampled. Land use types included soils under trees, uncropped soils, cropped soils at the pasture phase of a mixed rotation, cropped soils at the crop phase of a mixed rotation, and flooded soils. Slight differences in organic P stocks were found among land uses. Organic P was 21–35% lower in flooded soil than in the other treatments in the 0–100 cm depth. Inorganic P was significantly lower (ca. 27%) in pasture and cropped soils than in the uncropped controls at 0–25 cm depth. The ratios of organic P/inorganic P and organic C/organic P decreased with depth and did not significantly differ among the sites. The influence of cultivation on inorganic P to a depth of 100 cm depended on the initial phosphorus content of the soil. Soils rich in phosphorus lost substantial amounts of their phosphorus stocks, in some cases losses were as high as 70%, whereas phosphorus-poor soils presented only small changes in their inorganic P levels.

Effects of fertiliser on phosphorus pools in soils with contrasting organic matter content: A fractionation and path analysis study

With the intensification of agricultural production in many European countries, more marginal soils with elevated organic matter (OM) content are being brought into cultivation. However, little is known about the transformations in the constituent phosphorus (P) pools of organic soils receiving applications of P fertiliser. Soil P fractions were measured before and after receiving fertiliser in a controlled experiment to determine the change in the soil pools and path analysis was used to evaluate the relationships between P pools. In this study, P deficient soils ranging in OM content from 8 to 76%, were placed in large pots, planted with ryegrass and subjected to P fertiliser applications ranging from 10 to 145 kg ha−1, and monitored over an eight-month study period. High OM soils had a diminished ability to build-up the labile pool from freshly applied P, with relatively low increases up to 200% of the initial value, compared to mineral soils in which the labile pool increased to >2500% of the initial concentration. Additionally, organic soils had higher P uptakes in the grass yield than mineral soils, indicating a higher availability of added P in the soil solution than mineral soils due to their limited sorption ability. In general, there was a reduction in the organic P pool over applications from 0 to 55 kg ha−1, which was indicative of partial mineralization, but was followed by an accumulation of added P over applications from 55 to 145 kg P ha−1. The residual P pools did not build-up with P additions, but data indicated the occurrence of mineralization in most of the soils with decreases of around 40% of the initial concentrations. Organic and residual pools therefore displayed potential to supply P to more labile P pools across all soils of this study. Path analysis indicated that applied P was the only source of labile P in the soil with the highest OM content, leaving it dependent on continuous P applications to supply P for productivity, whereas in the rest of the soils there were interrelations between the non-labile and labile pools. Low pH strongly immobilised the applied P and should be corrected before the initiation of any fertilisation program, even in soils deficient in plant available P. The results demonstrated that P added as fertiliser to organic soils does not accumulate as in mineral soils, which may leave them susceptible to P losses in surface runoff. Therefore, organic soils under agricultural production located in high status catchments should receive low P applications and only during periods with low probability of precipitation to minimise the possibility of P exports to receiving waters.

Coupling of a nitrate production model with HYDRUS to predict nitrate leaching

Losses of nitrogen fertilizer from agricultural watersheds are among the main causes of water quality degradation in Quebec. Under maize crop, leached nitrate can reach up to 185 kg N ha−1. The synchronization of the nitrogen fertilization with crop uptake would be a way to optimize the use of nitrogen fertilizer and reduce nitrate leaching. Coupling an empirical nitrogen cycle model with a physical soil water dynamics model could simplify the optimization of nitrogen fertilizer use. The objective of this study was to couple the soil surface empirical nitrate production model adapted to the regional agro-climatic conditions of study area with HYDRUS to determine the most suitable combination in order to predict nitrate leaching into subsurface drains.In this study, we used estimation equations of nitrogen pools and transformations in nitrate. The equations, using historical field data, were combined into 60 N-release patterns taking into account a dissolution function of nitrate, 4 transformation and dissolution functions for the ammonium, 5 organic nitrogen pools and 3 N organic nitrogen from the soil and manure release functions. These N release patterns were applied in a mineral fertilizer treatment and four organic fertilizer treatments for a total of 300 unique nitrate release patterns.The results demonstrate the potential of a method to evaluate the contributions of nitrogen based on atmospheric data, crop rotation or soil particle sizes as 5% of the 300 combination gave a Nash–Sutcliffe efficiency coefficient (NSE) above 0.70. The prediction method must be different according to the source of the fertilization, organic or mineral as only 1% of the combination gave NSE above 0.70 in more than one treatment. In addition, the study showed that the soil organic nitrogen contribution to nitrate leaching tends to decrease as the organic fertilizer applications rate increases as 75% of the combination giving NSE above 0.70 in high fertilization treatment used the lower organic N pool and only 17% used the lower organic N pool for the agronomic fertilization rate and mineral fertilizer. The simulations also showed that the nitrate-leached masses are closely related to the fertilizer supply nitrogen content.

δ15N patterns in three subtropical estuaries show switch from nitrogen “reactors” to “pipes” with increasing degradation

AbstractOngoing alterations to estuaries by inland agricultural intensification and coastal development could affect their capacity to regulate the flux of excess terrestrial nitrogen (N) to the coastal ocean. Here, a new multiform δ15N metric was developed to measure how “pristine,” moderately impacted, and highly degraded estuaries recycle (assimilation, mineralization) and remove (denitrification, anaerobic ammonium oxidation) N. Organic (dissolved and particulate, δ15N and δ13C) and inorganic (nitrate and ammonium, δ15N and δ18O) N forms were measured over the salinity gradient in the wet and dry season in subtropical estuaries receiving increasing terrestrial N loads (pristine: 16 kg N d−1, moderate: 150 kg N d−1, degraded: 630 kg N d−1). The difference in the inorganic vs. organic pool δ15N composition increased between the pristine (0 ± 2‰), moderate (10 ± 6‰), and degraded (20 ± 8‰) systems, indicating that N recycling decreased as degradation increased. The N2O concentrations, NO3− dual isotope values, and offsets between “measured” and “mixing expected” δ15N values further revealed that microbial processes removed up to 30% of the N load entering the moderately degraded estuary, but only 9% in the highly degraded estuary. Hydrologic differences (depth and flushing times [FTs]) could not fully explain these shifts in N fate between the estuaries and seasons, which instead aligned with nonlinear increases in phytoplankton biomass and light penetration with increasing N loads. These isotopic indicators provide direct evidence that estuaries switch from “reactors” that assimilate and remove terrestrial N to “pipes” that transport N directly to sea as degradation increases.