Quality Of Organic Matter Inputs Research Articles

The effects of forage grasses and legumes on subsequent potato yield, nitrogen cycling, and soil properties

To balance economic potato production with environmental risks, it is crucial to understand the effects of forage crops preceding potatoes on soil nitrate (NO 3 - ) leaching potential, soil quality and potato yield. This study compared a legume forage (red clover, RC), a grass forage (timothy, T), and a red clover-timothy mixture (M) over two cycles of a 3-yr rotation (barley underseeded with forage-forage-potato) from 2013 to 2018. The forage crops were grown under low input maintenance system receiving only starter N fertilizer during the establishment year. Legume-based forages (i.e., RC and M) had greater dry matter accumulation and lower C:N ratio, and therefore added a greater quantity and quality of organic matter inputs to the soil compared with the grass-based forage (T). This resulted in increased soil N supply to the subsequent potato crop, and improved soil quality as indicated by increased aggregate stability, permanganate oxidizable carbon (POX-C), autoclaved citrate extractable soil protein (ACE), and flush of CO 2 upon rewetting a dried soil for legume- than grass-based forages. Under N limited conditions (i.e., no N fertilizer applied) forage legumes were associated with higher potato yield than forage grass. However, tuber yield was not increased when N fertilizer was applied, a finding attributed to a better synchrony between potato N uptake and soil N supply in the presence of N fertilizer with the forage grass treatment. The legume-based forages improved soil quality, increased soil N supply as well as NO 3 - leaching losses. Results of this study suggest that there may be a trade-off between selecting forage crops to reduce NO 3 - leaching and to enhance soil quality. • Compared forage legumes and grasses on soil quality, N cycling, and potato yield. • Soil quality improved, soil N supply and nitrate leaching increased with legumes. • Higher potato yield with legumes than grasses only under N limiting conditions. • Trends towards higher potato yield with forage grasses with N fertilizer applied. • Trade-offs between improving soil quality and potato yield and reducing N leaching.

Effects of harvest residue treatments on soil phosphorus fractions and availability in a young Chinese fir plantation

Forest harvesting changes the quantity and quality of organic matter inputs into soil, and thus would alter soil nutrient content and availability. Phosphorus (P) is a key element affecting plant growth. The effects of harvest residue treatments on soil P fractions and availability had not yet been evaluated. In this study, harvest residue retainment (RR), residue removal (R) and residue burning (RB) treatments were manipulated after clear-cutting in a mature Chinese fir (Cunninghamia lanceolata) plantation at the Sanming Forest Ecosystem and Global Change Research Station in Fujian, China. This study focused on the dynamics of soil P fractions and their driving factors in the 0-10 cm and 10-20 cm soil layers after 4-year residue treatments. The results showed that, in RR treatment, the contents of easily-available P, moderately-available P and non-available P at the 0-10 cm soil layer were all significantly higher than those in R treatment, while the contents of moderately-available P and non-available P at the 10-20 cm soil layer was significantly higher than those in RB treatments. The ratios of soil organic carbon (C) to organic P (C:Po) in both layers were over 200 for all the three treatments, with ratios in RR treatment being significantly lower than those in RB and R treatments, indicating that RR could alleviate P limitation in this ecosystem. Moreover, results of the redundancy analysis showed that changes in P fractions were mainly affected by dissolved organic C, free Fe and noncrystalline amorphous Fe. The results suggested that soil organic P and available P were mainly from the decomposition of plant residues, which supported continuous P supply for plant growth. RR could enhance soil P content, thereby improve soil P availability and mitigate P limitation in Chinese fir plantation.

Modelling of mangrove annual leaf litterfall with emphasis on the role of vegetation structure

Understanding local patterns of species-specific litterfall can help predict spatial differences in the amount and quality of organic matter input to systems under different environmental conditions. However, little is known about the drivers of differences in litterfall production in mangroves. Here we combined data from leaf litterfall (LL) studies (n = 13) and floristic studies (n = 16) conducted in the mangroves of the Ajuruteua peninsula (North of Brazil), at sites composed mainly of Rhizophora mangle and/or Avicennia germinans. We investigated the relationship between LL and vegetation parameters (height, diameter, basal area, density, and relative density) using partial least-squares regressions (PLS-R), building four models with different combinations of these parameters. Vegetation parameters explained 69–85% of the annual LL of A. germinans and 50–66% of R. mangle. Relatively strong, univariate significant relationships were also found between basal area and A. germinans LL (R2 = 0.40), and between height and R. mangle LL (R2 = 0.41). Nonetheless, models with four to five predictors yielded better fits. To illustrate the applicability of our approach we used the PLS-R models to calculate the LL of simulated sites. This simulation indicated a clear increase in LL production along the transition from Avicennia- to Rhizophora-dominated mangroves. As the leaf litter of these species differ substantially in chemical composition and decomposability, such transition in species composition will likely affect nutrient dynamics in the region. Thus, we suggest reporting species-specific site characteristics along with LL in future studies to obtain more robust models to predict future LL production at sites of changing vegetation structure.

Managing the value of composts as organic amendments and fertilizers in sandy soils

Nutrient rich composts are employed at low rates to minimize risks of N and P losses; this limits their value as soil improvers through C addition and the build up of soil organic matter. Blending with nutrient-poor composts such as those from the organic fraction of municipal solid waste could reduce the risks of nutrient losses while maintaining the positive effects on soil organic matter. We conducted a 2-yr experiment with composts of diverse origin: organic fraction of municipal solid waste (MC), cattle feedlot manure (FC), poultry litter (PC) and biosolids (BC), alone or blended (FC-MC, PC-MC) in a sandy soil under the humid warm climatic conditions of NE Argentina. We studied the effects of a single application (40 Mg ha−1) on the surface soil (0–10cm) properties of a permanent subtropical pasture through annual chemical and biological analyses. On five dates, available N and P were also determined at 0–10cm and 55–65cm. Soil total C and N increased over time while potential N mineralization and CO2 emission decreased. All amendments resulted in similar increments of soil C and N despite marked differences in quantity and quality of organic matter inputs. Because MC had substantial amounts of Ca carbonates, it contributed to a reduction of available P from manure composts through dilution and precipitation. The release of available P from biosolids composts (where P is bound to Fe and Al) was lower than from manure composts (where P is bound by Ca phosphates). The highest environmental risk from compost application would likely be the leaching of soluble N produced during the composting process and released immediately after field application. Blending with N-poor MC would contribute to nitrate dilution.

Long-term effects of elevated CO2 on carbon and nitrogen functional capacity of microbial communities in three contrasting soils

Elevated atmospheric CO2 (eCO2) affects soil-plant systems by stimulating plant growth, rhizosphere processes and altering the amount and quality of organic matter inputs. This study examined whether these plant-mediated processes indirectly influence the structure and function of soil microbial communities and soil carbon (C) and nitrogen (N) cycling. Surface soils (0–5 and 5–10 cm) of Calcarosol, Chromosol and Vertosol were sampled after 5 years' exposure to either ambient CO2 (aCO2; 390 ppm) or eCO2 (550 ppm) using free-air CO2 enrichment (SoilFACE). Changes in microbial community structure were not detected using automated ribosomal intergenic spacer analyses (ARISA). However, quantitative PCR of targeted organic C decomposition (cu, cbh), N mineralisation (apr, npr), nitrification (amoB, amoA, norA) and denitrification (nirK, narG, nosZ) genes showed that eCO2 reduced the abundance of half of the functional genes in the Chromosol and Vertosol and their abundance was tightly coupled with total C and N pools. In the Chromosol, total N and C of soil (<2 mm particles) was reduced by up to 20% and was associated with enhanced microbial activity (soil respiration). Soil C was also reduced in the Vertosol (15%, 5–10 cm); however greater laccase, reduced cellulase and lower microbial activity indicated that organic matter decomposition was currently limited by N. The loss of soil organic N and C under eCO2 was likely driven by greater N demand. This study highlighted that the indirect effects of eCO2 on functional capacity of soil microbial communities in dryland agricultural system depended on the soil type.

Plant community structure regulates responses of prairie soil respiration to decadal experimental warming.

Soil respiration is recognized to be influenced by temperature, moisture, and ecosystem production. However, little is known about how plant community structure regulates responses of soil respiration to climate change. Here, we used a 13-year field warming experiment to explore the mechanisms underlying plant community regulation on feedbacks of soil respiration to climate change in a tallgrass prairie in Oklahoma, USA. Infrared heaters were used to elevate temperature about 2°C since November 1999. Annual clipping was used to mimic hay harvest. Our results showed that experimental warming significantly increased soil respiration approximately from 10% in the first 7years (2000-2006) to 30% in the next 6years (2007-2012). The two-stage warming stimulation of soil respiration was closely related to warming-induced increases in ecosystem production over the years. Moreover, we found that across the 13years, warming-induced increases in soil respiration were positively affected by the proportion of aboveground net primary production (ANPP) contributed by C3 forbs. Functional composition of the plant community regulated warming-induced increases in soil respiration through the quantity and quality of organic matter inputs to soil and the amount of photosynthetic carbon (C) allocated belowground. Clipping, the interaction of clipping with warming, and warming-induced changes in soil temperature and moisture all had little effect on soil respiration over the years (all P>0.05). Our results suggest that climate warming may drive an increase in soil respiration through altering composition of plant communities in grassland ecosystems.

Deposit‐feeder diets in the Bering Sea: potential effects of climatic loss of sea ice‐related microalgal blooms

Climate warming in seasonally ice-covered seas is expected to reduce the extent and duration of annual sea ice. Resulting changes in sea ice related blooms of ice algae or phytoplankton may in turn alter the timing, magnitude, or quality of organic matter inputs to the sea floor. If benthic taxa rely differently on direct consumption of settling fresh microalgae for growth and reproduction, altered blooms may lead to reorganization of deposit-feeding assemblages. To assess the potential for such changes, we examined the diets of five abundant deposit-feeders (three infaunal bivalves, a polychaete, and a brittle star) with different feeding modes over the course of the spring bloom in May–June 2007 in the north-central Bering Sea (30–90 m depth). Short-term data from gut contents reflected feeding modes, with the bivalves Macoma calcarea, Ennucula tenuis, and Nuculana radiata, and the brittle star Ophiura sarsi, responding more quickly to deposition of fresh algae than did the head-down polychaete Pectinaria hyperborea. Fatty acid biomarkers also indicated rapid ingestion of settling algae by the bivalves (especially Macoma) and the brittle star, while Pectinaria continued to ingest mainly bacteria. Fatty acid biomarkers did not indicate any unique dietary importance of ice algae released from melting ice. Longer-term inference from stable isotopes suggested that fresh microalgae contributed little to overall carbon assimilated by any of these species. Instead, deposit-feeders appeared to select a consistent fraction from the pool of sediment organic matter, probably heterotrophic microbes, microbial products, and reworked phytodetritus that form a longer-term sediment “food bank.” Redistribution of settled organic matter via scouring and accumulation by currents, as well as the multi-year life spans of macroinvertebrates, may further overwhelm effects of short-term variations in the timing, magnitude, and dispersion of blooms in the water column. More diet data are needed from midsummer to account for any lag in assimilation of fresh microalgae at these cold temperatures. Nevertheless, our results suggest that if annual sea ice cover is reduced, increased production of phytoplankton during longer ice-free periods could replace inputs of ice-associated microalgae to the sediment food bank used by deposit-feeders.

Riverine transport of terrestrial organic matter to the North Catalan margin, NW Mediterranean Sea

Rivers are the primary pathway for organic matter transport from the terrestrial to the marine environment and, thus, river fluxes are critical in regulating the quantity of terrestrial organic matter that reaches the coastal ecosystems. Hydrodynamic processes typical of the coastal zone can lead to the transport of terrestrial organic matter across the continental shelf and beyond. Such organic matter can eventually reach the deep margin and basin ecosystems. Riverine inputs of organic matter to the sea can be a significant food source to marine ecosystems contributing to carbon cycling in these ecosystems. In order to assess the marine carbon cycle it is essential to know the biogeochemical characteristics and temporal dynamics of the fluvial organic matter input discharged by rivers to the coastal zone. In this study we present a one and a half year long (November 2008 to May 2010) assessment on organic carbon (OC) and nitrogen (N) inputs from the three main rivers discharging into the North Catalan margin (Tordera, Ter and Fluvià, from south to north). Furthermore, we investigate the characteristics of the particulate organic matter discharged by these rivers by means of stable isotopic (δ13C and δ15N) and grain size analyses.We found that the hydrological regime of the rivers is a relevant factor in regulating the quantity and mediating the quality of organic matter inputs to the North Catalan margin. Overall, the three main rivers discharging into the study area deliver 1266 and 159 tonnes of terrestrial OC and N per year, respectively, to the coastal zone. Most of the OC and N load is transported during floods, which indicates that the Mediterranean climate of the area, with a strong seasonal contrast in precipitation, determines the timing of the main inputs of OC and N to the sea. Therefore, the annual OC and N load experiences a high temporal variability associated to the number and magnitude of floods with in each hydrological year. In addition, we found that water reservoirs along the rivers act as traps for terrestrial organic matter, reducing its delivery and ultimate burial into marine sediments. River hydrology also affects the quality of organic matter that reaches the coastal zone (both in terms of C and N) by shifting the relative weight of the various sources of terrestrial organic matter. During low river discharge (i.e., in summer and early autumn) the main contributor to the organic matter pool is mostly associated with freshwater primary producers, whereas with relatively high water flows (i.e., in winter and spring) the main contributor is associated with erosion and release of soil organic matter. Furthermore, the impact of waste water treatment plants into the studied rivers results in the alteration of the isotopic signal of suspended N. The three studied rivers play a major role in transporting terrestrial organic matter to the North Catalan margin, but the fraction that is exported to the deep margin by high-energy episodic hydrodynamic events, such as large coastal storms, has a minor importance.

Alpine grassland soils contain large proportion of labile carbon but indicate long turnover times

Abstract. Alpine soils are expected to contain large amounts of labile carbon (C) which may become a further source of atmospheric carbon dioxide (CO2) as a result of global warming. However, there is little data available on these soils, and understanding of the influence of environmental factors on soil organic matter (SOM) turnover is limited. We extracted 30 cm deep cores from five grassland sites along a small elevation gradient from 2285 to 2653 m a.s.l. in the central Swiss Alps. Our aim was to determine the quantity, allocation, degree of stabilization and mean residence time (MRT) of SOM in relation to site factors such as soil pH, vegetation, and SOM composition. Soil fractions obtained by size and density fractionation revealed a high proportion of labile C in SOM, mostly in the uppermost soil layers. Labile C in the top 20 cm across the gradient ranged from 39.6–57.6 % in comparison to 7.2–29.6 % reported in previous studies for lower elevation soils (810–1960 m a.s.l.). At the highest elevation, MRTs measured by means of radiocarbon dating and turnover modelling, increased between fractions of growing stability from 90 years in free POM (fPOM) to 534 years in the mineral associated fraction (mOM). Depending on elevation and pH, plant community data suggested considerable variation in the quantity and quality of organic matter input, and these patterns could be reflected in the dynamics of soil C. 13C NMR data confirmed a relationship of SOM composition to MRT. While low temperature in alpine environments is likely to be a major cause for the slow turnover rate observed, other factors such as residue quality and soil pH, as well as the combination of all factors, play an important role in causing small scale variability of SOM turnover. Failing to incorporate this interplay of controlling factors into models may impair the performance of models to project SOM responses to environmental change.

Soil Microbial Biomass And Activity In A Cork Oak Savanna

Cork oak savannas are composed by a sparse tree canopy (30-70 trees/ha) and a grassland understory predominantly composed of C3 annuals that survive the hot and dry Mediterranean summers as seeds in the soil. Microbial communities can be more or less efficient at converting organic substrates into microbial biomass carbon depending on the quantity and quality of organic matter inputs. The cork oak savannas have two distinct types of plant litter that can affect soil microbial biomass and activity differently: herbaceous litter and the more recalcitrant woody plant litter resulting from the trees. Spatial variability of soil microbial biomass and activity due to the tree-grassland component of cork oak savannas were evaluated in order to better understand the soil carbon dynamics of these systems.To quantify changes in soil microbial biomass and activity, measurements were performed in a Cork oak savanna in Southern Portugal. At this site 8 plots were randomly established under mature cork oak trees and paired with 8 open grassland plots. During one year soil cores (0-10 cm) were monthly collected at each site for measuring soil microbial biomass C and other eco-physiology parameters. Soil microbial biomass carbon (Cmic) and nitrogen (Nmic) were always higher under the tree canopy than in the open grasslands. Organic carbon (Corg) was also higher under the tree canopies. The Cmic/Corg ratio relates to the microbial activity and its potential to mineralize organic substances. The Cmic/Corg ratio was lower under the tree canopies than in the open grasslands. Less microbial biomass was supported per unit of Corg. Basal activity was always higher under the canopy than in the open grassland.Trees scattered in the savanna function as islands inducing larger soil microbial communities and higher basal activity under the canopies. Lower Cmic/Corg ratio under the tree canopies suggests a more recalcitrant nature of the litter and a decrease in relative availability of organic matter under the trees.

Soil Microbial Biomass And Activity In A Cork Oak Savanna

AbstractCork oak savannas are composed by a sparse tree canopy (30-70 trees/ha) and a grassland understory predominantly composed of C3 annuals that survive the hot and dry Mediterranean summers as seeds in the soil. Microbial communities can be more or less efficient at converting organic substrates into microbial biomass carbon depending on the quantity and quality of organic matter inputs. The cork oak savannas have two distinct types of plant litter that can affect soil microbial biomass and activity differently: herbaceous litter and the more recalcitrant woody plant litter resulting from the trees. Spatial variability of soil microbial biomass and activity due to the tree-grassland component of cork oak savannas were evaluated in order to better understand the soil carbon dynamics of these systems.To quantify changes in soil microbial biomass and activity, measurements were performed in a Cork oak savanna in Southern Portugal. At this site 8 plots were randomly established under mature cork oak trees and paired with 8 open grassland plots. During one year soil cores (0-10 cm) were monthly collected at each site for measuring soil microbial biomass C and other eco-physiology parameters. Results/Conclusion Soil microbial biomass carbon (Cmic) and nitrogen (Nmic) were always higher under the tree canopy than in the open grasslands. Organic carbon (Corg) was also higher under the tree canopies. The Cmic/Corg ratio relates to the microbial activity and its potential to mineralize organic substances. The Cmic/Corg ratio was lower under the tree canopies than in the open grasslands. Less microbial biomass was supported per unit of Corg. Basal activity was always higher under the canopy than in the open grassland.Trees scattered in the savanna function as islands inducing larger soil microbial communities and higher basal activity under the canopies. Lower Cmic/Corg ratio under the tree canopies suggests a more recalcitrant nature of the litter and a decrease in relative availability of organic matter under the trees.

Brassicaceous and rye cover crops altered free-living soil nematode community composition

Nematode community analysis was utilized to evaluate the biofumigant or allelopathic effects of brassicaceous and rye winter cover crops on non-target nematodes in three experiments (two sites) in Maryland. The cover crop treatments included mustard blend ( Sinapis alba and Brassica juncea) ‘Caliente’, rapeseed ( B. napus) ‘Essex’/’Humus’, forage radish ( Raphanus sativus) ‘Dichon’, oilseed radish ( R. sativus) ‘Adagio’/’Colonel’, rye ( Secale cereale) ‘Wheeler’ and a no cover crop (winter weeds) control. Soil samples (0–15 cm) were collected two or three times per year and extracted nematodes were identified to genus or family. Nematode response parameters were genus, family, trophic group population density, and percent distribution of trophic groups in the entire nematode community divided into colonizer-persister ranks. The parameters refer only to free-living nematodes, however facultative hyphal-root hair feeding Tylenchidae were included. Cover crops had unique impacts on nematode communities, but these impacts appeared to be associated more with quality of organic matter inputs rather than biofumigation or allelopathy. Across all dates and seasons, and four to nine months after winter-kill, dormant bacterivore (dauer larvae) nematode populations in the forage radish (C/N shoots ∼10) plots ranged from 3.5 to 15.7 times higher ( P < 0.10) than in the control plots. Plant-associate (Tylenchidae) nematodes were 4–6.5 times higher ( P < 0.10) in rapeseed or rye (C/N shoots ∼25) plots compared to the control in June of two experiments. Across experiments fungivore nematode abundance was increased in either rapeseed ‘Essex’ or rye compared to radishes or the control. Correlations of nematode community groups with cover crop and soil parameters suggested that dauer larvae abundance was associated with soil moisture in radish plots, and tissue quality and quantity at the time of cover crop termination was associated with nematode community response. Canonical discriminant analysis suggested that rapeseed and rye had similar effects on the nematode community composition, as did the two radish cultivars, though distinct from the effect of rye and rapeseed. Overall, results suggest that radishes stimulated a bacterial decomposition pathway, while rapeseed and rye stimulated a proportionally greater fungal-based food web.

Soil organic matter in soil physical fractions in adjacent semi-natural and cultivated stands in temperate Atlantic forests

Changes from natural tree species to rapidly growing exotic species as well as intensification of forestry operations with heavy machinery can lead to changes in the quantity and quality of organic matter inputs to soil and to disruption of soil physical structure. These two ecosystem properties are tightly linked to organic matter dynamics. Five adjacent forest stands were selected to study soil organic matter dynamics in soil physical fractions. On one hand, two semi-natural broadleaved forests ( Quercus robur, Fagus sylvatica) and an adult radiata pine plantation (40-year-old,) in order to study the effect of species change on these parameters, and on the other, a chronosequence of Pinus radiata plantations (40-year-old; 3-year-old; 16-year-old), to study the effect of mechanization during harvesting and intense site preparation. Samples of intact topsoil (0–5 cm) were collected and aggregate-size distribution, mean weight diameter (MWD), total C and N, particulate organic matter (POM)-C, POM-N and microbial biomass-C were determined in each aggregate size fraction. Microbial respiration and nitrogen mineralization were also assessed in each aggregate size fraction, during a 28 day incubation period. Losses of POM-C and POM-N in the bulk soil due to mechanical site preparation were high relative to total soil C and N, which suggests that POM is a sensitive parameter to the effect of mechanization. The ratio C-POM:SOM was significantly related to MWD ( R 2 = 0.75, P < 0.001) reflecting that POM may play a key role in the topsoil aggregate formation in these stands. Semi-natural stands had a higher proportion of macroaggregates (0.25–2 mm) than the cultivated adult one. Megaaggregates (>2 mm) were the most abundant class in mature stands (82–92%), whereas macro- and microaggregates (<2 mm) were the most abundant ones in the intensely soil prepared P. radiata plantation (49%). Indicators for sustainable forest management related to soil organic matter should not only be assessed in terms of total C stocks but also with respect to sensitive organic matter and its degradability in different size classes.

Conservation Agriculture and Soil Carbon Sequestration: Between Myth and Farmer Reality

Improving food security, environmental preservation and enhancing livelihood should be the main targets of the innovators of today's farming systems. Conservation agriculture (CA), based on minimum tillage, crop residue retention, and crop rotations, has been proposed as an alternative system combining benefits for the farmer with advantages for the society. This paper reviews the potential impact of CA on C sequestration by synthesizing the knowledge of carbon and nitrogen cycling in agriculture; summarizing the influence of tillage, residue management, and crop rotation on soil organic carbon stocks; and compiling the existing case study information. To evaluate the C sequestration capacity of farming practices, their influence on emissions from farming activities should be considered together with their influence on soil C stocks. The largest contribution of CA to reducing emissions from farming activities is made by the reduction of tillage operations. The soil C case study results are not conclusive. In 7 of the 78 cases withheld, the soil C stock was lower in zero compared to conventional tillage, in 40 cases it was higher, and in 31 of the cases there was no significant difference. The mechanisms that govern the balance between increased or no sequestration after conversion to zero tillage are not clear, although some factors that play a role can be distinguished, e.g., root development and rhizodeposits, baseline soil C content, bulk density and porosity, climate, landscape position, and erosion/deposition history. Altering crop rotation can influence soil C stocks by changing quantity and quality of organic matter input. More research is needed, especially in the tropical areas where good quantitative information is lacking. However, even if C sequestration is questionable in some areas and cropping systems, CA remains an important technology that improves soil processes, controls soil erosion and reduces production cost.

Heterotrophic nitrogen fixation in oligotrophic tropical marshes: changes after phosphorus addition

In order to determine the impact of nutrient enrichment on phosphorus (P) limited wetlands, we established experimental P additions in marshes throughout northern Belize. P significantly increased macrophyte primary production, which led to the rapid elimination of cyanobacterial mats. The replacement of cyanobacterial mats by macrophytes constrained autotrophic nitrogen (N) fixation, increased the quantity, and changed the quality of organic matter input to the sediments. We predicted that the activity of sediment heterotrophic N fixers will be impacted by these alterations in carbon input. We used the acetylene reduction technique to measure potential (glucose amended) nitrogenase activity (NA) in sediments from controls and treatment plots that have been P enriched for four years and dominated either by Eleocharis cellulosa, or Typha domingensis for two years. NA in P-enriched plots was 2–3 orders of magnitude higher than NA in controls. NA was positively correlated with the soil reactive P, both total organic and microbial carbon, live root biomass, and total phospholipid fatty acids (PLFA) as an indicator of active microbial biomass. It was negatively correlated with the concentration of ammonium-N. Path analysis revealed that the indirect effect of P on NA through the root biomass was more important than the direct effect of P. NA of the upper sediment layer was consistently higher in Eleocharis than in Typha dominated plots, despite the higher litter input by Typha. We feel that the higher levels of lignin and phenolics occurring in Typha litter, relative to Eleocharis, constrained NA in Typha plots.

Predictors of gross N mineralization and immobilization during decomposition of stabilized organic matter in agricultural soil

SummaryLong‐term additions of different types of organic amendments affect the amount of soil organic matter. Less is known about how this in turn affects carbon (C) and nitrogen (N) mineralization from the pool of stabilized soil organic matter, or the extent to which gross N immobilization influences the net amount of N mineralized. Soils, differing in the quantity and quality of organic matter inputs they had received since 1956, were sampled approximately 6 or 18 months after the most recent applications of organic amendments. Two laboratory experiments were carried out to: (i) evaluate if, and how, the organic amendments had affected C mineralization, gross and net N mineralization; (ii) examine the relation between gross N immobilized and free‐light fraction of organic matter; and (iii) assess predictors for gross N mineralization and immobilization rates in soils. The amount of soil organic C and N were major determinants of C and gross N mineralization, but not of net N mineralization. Carbon mineralization was related to gross N mineralization, but the ratio between the two was not constant. Gross N immobilization was related to the amount of free‐light fraction material in the soil with 90% variation explained. For most common organic amendments applied in autumn, our results support the use of total soil organic N and C mineralization as predictors of gross N mineralization from stabilized soil organic matter. In addition, we propose that the amount of free‐light fraction material present in the soil in spring is adequate as a predictor of the immobilization potential of the soil, without a need to consider the C‐to‐N ratio of this material.

Studies on litter characterization using 13C NMR and assessment of microbial activity in natural forest and plantation crops’ (teak and rubber) soil ecosystems of Kerala, India

The leaf litter is the major source of soil organic matter in natural and many plantation crop ecosystems. Quantity and quality of organic matter in a soil ecosystem is of utmost importance in regulating the soil health. Hence assessment of quality of organic matter input, viz., litter is important and is attempted in this study. The leaf litter of rubber (Hevea brasiliensis), pueraria (Pueraria phaseoloides), mucuna (Mucuna bracteata), teak (Tectona grandis) and forest (mixed species) were analyzed using solid state 13C nuclear magnetic resonance (NMR) to study the relative abundance of different carbon compounds present. The spectra revealed that litter of all species studied contain relatively larger amounts of polysaccharides compared to other C containing compounds. Also it could be observed that the alkyl-C to O-alkyl-C ratio of rubber litter was much higher compared to that of others. Aromatics and carbonyl compounds were also present in all litter species. The resource quality based on alkyl-C to O-alkyl-C ratio of the litter samples studied can be arranged in the order pueraria > teak > mucuna > forest > rubber. The respiration rate, substrate induced respiration rate and biomass-C (Cmic) of the litter samples were estimated. It could be observed that litter associated microbial activity decreased as alkyl-C to O-alkyl-C ratio increased. Resource quality derived from the NMR spectra and the litter biological properties were complementary. Soil samples (0–15 cm) from the five soil ecosystems (rubber, pueraria, mucuna, teak and forest) were analyzed for respiration rate, substrate induced respiration rate, Cmic, total-C and total-N. The forest soil had higher respiration rate, total-C and total-N compared to cultivated soil systems. Pueraria, mucuna and teak soils were comparable for their biological properties while rubber soil recorded comparatively lower microbial activity.

Soluble organic nitrogen pools in adjacent native and plantation forests of subtropical Australia

Soil soluble organic nitrogen (SON) can play an important role in soil nitrogen (N) cycling in forest ecosystems. This study examined the effect of land-use change from a native forest (NF) to a first rotation (1R) and subsequent second rotation (2R) hoop pine ( Araucaria cunninghamii) plantation on soil SON pools. The impact of residue management on SON pools was also investigated in the 2R forest, where SON was measured in tree rows (2R-T) and windrows (2R-W). Various extraction techniques were used to measure SON pool size in the 0–10, 10–20 and 20–30 cm layers of soil. The results showed that land-use change had a significant impact on soil SON pools. In the 0–10 cm layer, 3.2–8.7, 14–23, 20–28, 60–160 and 127–340 mg SON kg −1 were extracted by water, 0.5 M K 2SO 4, 2 M KCl, hot water and hot 2 M KCl, respectively. The size of the SON pools and the potential production of SON (PPSON) were generally highest in the NF soil and lowest in the 2R-T soil, and in all forest types decreased with soil depth. The larger SON pools in the NF soil coincided with lower soil, litter and root C:N ratios, suggesting that the difference in the size of SON pools between the NF and 1R soil may be related to differences in the quality of organic matter input under the different forest ecosystems. Differences in the size of SON pools between the 1R soil and the 2R soils and between the 2R-T soil and the 2R-W soil may be related to the quantity of organic matter input and time since disturbance. Significant relationships were found between the SON extracted by 0.5 M K 2SO 4 (SON ps) and 2 M KCl (SON KCl), and also among the SON extracted by hot 2 M KCl (SON hKCl), hot water (SON hw) and water (SON w), suggesting that the organic N released by these groups of extracts may be at least partly from similar pools.

Soil Carbon and Nitrogen Storage in Response to Fire in a Temperate Mixed‐Grass Savanna

Vegetation fires may alter the quantity and quality of organic matter inputs to soil, rates of organic matter decay, and environmental factors that influence those processes. However, few studies have evaluated the impacts of this land management technique on soil organic carbon (SOC) and total N in grasslands and savannas. We evaluated the impact of repeated fires and their season of occurrence on SOC and total N storage in a temperate mixed-grass-mesquite savanna where fire is used to control woody plant encroachment. Four fire treatments varying in season of occurrence were examined: summer only (SF), winter only (WF), alternate summer and winter fires (SWF), and unburned controls. In each treatment, soils were sampled to 1 m under three vegetation types: C3 grasses, C4 grasses, and mesquite trees. The SOC storage at 0 to 20 cm was significantly greater in SF (2693 g C m(-2)) and SWF (2708 g C m(-2)) compared to WF (2446 g C m(-2)) and controls (2445 g C m(-2)). The SWF treatment also increased soil total N (271 g N m(-2)) relative to all other treatments (228-244 g N m(-2)) at 0 to 20 cm. Fire had no effect on SOC or total N at depths of > 20 cm. Vegetation type had no significant influence on SOC or total N stocks. The delta13C value of SOC was not affected by fire, but increased from -21 per thousand at 0 to 10 cm to -15 per thousand at depths of > 20 cm indicating that all treatments were once dominated by C4 grasses before woody plant encroachment during the past century. These results have implications for scientists, land managers, and policymakers who are now evaluating the potential for land uses to alter ecosystem C storage and influence atmospheric CO2 concentrations and global climate.

Soil Processes Affected by Sixteen Grassland Species Grown under Different Environmental Conditions

Plant species, and their interactions with the environment, determine both the quantity and chemistry of organic matter inputs to soils. Indeed, countless studies have linked the quality of organic matter inputs to litter decomposition rates. However, few studies have examined how variation in the quantity and chemistry of plant inputs, caused by either interspecific differences or changing environmental conditions, influences the dynamics of soil organic matter. We studied the effects of 16 grassland species from 4 functional groups (C3 and C4 grasses, forbs, and legumes) growing under ambient and elevated CO2 (560 ppm) and N inputs (4 g m−2 yr−1) on soil carbon (C) and nitrogen (N) dynamics after 4 yr in a grassland monoculture experiment in Minnesota, USA. Specifically, we related soil C and N dynamics to variation among species and their responses to the CO2 and N treatments in plant biomass and chemistry of roots, the dominant detrital input in the system. The 16 species caused much larger variation in plant litter inputs and chemistry, as well as soil C and N dynamics, than the CO2 and N treatment. Not surprising, variation in the quantity of plant inputs to soils contributed to up to a two‐fold variation in microbial biomass and amount of respired nonlabile soil C. Root N concentration (across species and CO2 and N treatments) was significantly negatively related to decomposition of nonlabile soil C and positively related to net N mineralization. Greater labile C inputs decreased rates of net N mineralization, likely because of greater N immobilization. Thus, of the traits examined, plant productivity, tissue N concentration, and labile C production such as from rhizodeposition were most important in causing variation in soil C and N dynamics among species and in response to altered atmospheric CO2 and N supply.