Influence of organic by-products and nitrogen source on chemical and microbiological status of an agricultural soil

We assessed the influence of the addition of four municipal or agricultural by-products (cotton gin waste, ground newsprint, woodchips, or yard trimmings), combined with two sources of nitrogen (N), [ammonium nitrate (NH₄NO₃) or poultry litter] as carbon (C) sources on active bacterial, active fungal and total microbial biomass, cellulose decomposition, potential net mineralization of soil C and N and soil nutrient status in agricultural soils. Cotton gin waste as a C source promoted the highest potential net N mineralization and N turnover. Municipal or agricultural by-products as C sources had no affect on active bacterial, active fungal or total microbial biomass, C turnover, or the ratio of net C:N mineralized. Organic by-products and N additions to soil did not consistently affect C turnover rates, active bacterial, active fungal or total microbial biomass. After 3, 6 or 9 weeks of laboratory incubation, soil amended with organic by-products plus poultry litter resulted in higher cellulose degradation rates than soil amended with organic by-products plus NH₄NO₃. Cellulose degradation was highest when soil was amended with newsprint plus poultry litter. When soil was amended with organic by-products plus NH₄NO₃, cellulose degradation did not differ from soil amended with only poultry litter or unamended soil. Soil amended with organic by-products had higher concentrations of soil C than soil amended with only poultry litter or unamended soil. Soil amended with organic by-products plus N as poultry litter generally, but not always, had higher extractable P, K, Ca, and Mg concentrations than soil amended with poultry litter or unamende soil. Agricultural soil amended with organic by-products and N had higher extractable N, P, K, Ca and Mg than unamended soil. Since cotton gin waste plus poultry litter resulted in higher cellulose degradation and net N mineralization, its use may result in faster increase in soil nutrient status than the other organic by-products and N sources that were tested. Received: 15 May 1996     

Recycling Newsprint on Agricultural Land with the Aid of Poultry Litter

The large volume of cellulose products, primarily newsprint and yard waste, that are disposed of in landfills and the resistance of these products to decomposition led to an evaluation of the disposal of ground newsprint on agricultural land. A field study was conducted with cotton to evaluate the effects of trenching and mixing the excavated soil with ground newsprint and/or poultry litter. Decomposition rate of newsprint and potential for environmental contamination were investigated in a Cahaba-Wickham-Bassfield sandy loam (Typic Hapludult) soil. The experimental variables included different ratios of soil, ground newsprint, and/or poultry litter applied on the soil surface and in trenches 0.61 m or 1.22 m deep. When ground newsprint and excavated soil were mixed without adjusting the C:N ratio of the backfill, the newsprint was still present seven months after application. However, adjusting the C.N ratio of the excavated soil and ground newsprint with poultry litter provided the nitrogen necessary to completely decompose the ground newsprint within seven months. Soil surface application of ground newsprint required adjusting the C:N ratio to control the occurrence of plant pathogenic organisms. Extractable soil nutrients were increased when poultry litter was added, including P and K which are of concern with respect to surface and groundwater contamination. Extractable Zn, Cu, and Mn levels were increased by the addition of poultry litter, but their levels were in the ranges that most row crops will tolerate. The metals Cr and Pb, found in some printer's ink, may be of concern if repeated application of newsprint is made to the same site. Soil organic matter content was increased from 11.9 g kg-1 to 23.8 g kg-1 in the 50:40:10 backfill mixture of soil, newsprint, and poultry litter seven months after application.     

Long‐term phosphorus solubility in soils receiving poultry litter treated with aluminum,calcium, and iron amendments

Abstract

Phosphorus (P) runoff from poultry litter applied to fields can adversely impact water quality. The majority of P in runoff from poultry litter is soluble, so decreasing the solubility of P could lessen the impact of poultry litter on water quality. The objective of this study was to determine long‐term P solubility in soils receiving poultry litter treated with aluminum (Al), calcium (Ca), and iron (Fe) amendments at various soil pHs. Soil pH was adjusted to 4.0, 5.0, 6.0, 7.0, and 8.0 using elemental sulfur (S) or CaCO₃ with some soil left at its native pH. The pH‐adjusted soil was then incubated with either no litter (control), litter alone (litter control), or litter amended with alum, A1₂(SO₄)₃.16H₂O, (100 or 200 g/kg), Ca(OH)₂ (25 or 50 g/kg), or FeSO₄ .7H₂O (100 or 200 g/kg). The soil was then allowed to equilibrate in the dark at room temperature for 0, 7, 49, 98, and 294 days. After equilibration, soils were extracted with deionized water and soluble reactive P levels were determined. Water‐soluble P levels decreased with time in all treatments, including the control and litter control treatments. Soil pH also affected soluble reactive P levels, with the lowest levels generally observed at pH 8.0. Addition of both unamended and chemically‐amended litter to soil significantly increased P concentrations at all combinations of pH and sampling time. Addition of chemically‐amended litter to soil significantly reduced soluble reactive P compared to unamended litter. With all treatments, an apparent equilibrium was reached at 98 d after treatment. Amendment of litter with either FeSO₄ .7H₂O or alum resulted in the lowest soluble reactive P levels after 294 days. Use of chemical amendments to limit P solubility has potential and should be pursued as a means of reducing eutrophication of sensitive surface waters where poultry litter is applied as a fertilizer.     

Impact of Animal Manure on Soil Chemistry,Mineral Nutrients,Yield, and Fruit Quality in ‘Golden Delicious’ Apple

ABSTRACT

Incorporating deep litter cow and deep litter poultry manures with the top 30-cm soil improved orchard soil chemistry, including nutrient availability, soil organic matter, electrical conductivity (EC), pH, cation exchange capacity (CEC) and biological activity in a ‘Golden Delicious’ apple (Malus domestica Bork) orchard in Zanjan, Iran. Application of deep litter cow manure at 30 t ha^?1 or deep litter poultry manure at 10 t ha^?1 resulted in a higher rate of nitrogen (N) release, and thus increased yield and fruit size, but decreased fruit color. The least leaf minerals were found in the untreated control trees. The control trees showed minor symptoms of N, iron (Fe), and magnesium (Mg) deficiencies during the following season. Positive correlation existed between the rate of manure applied and the content of soil organic matter (OM). The deep litter poultry manure at 10 t ha^?1 increased the soil K, Mg, calcium (Ca), ammonium-N, and EC levels.     

Plant canopy effects on litter accumulation and soil microbial biomass in two temperate forests

The objective of this study was to determine whether differences in canopy structure and litter composition affect soil characteristics and microbial activity in oak versus mixed fir-beech stands. Mean litter biomass was greater in mixed fir-beech stands (51.9t ha⁻¹) compared to oak stands (15.7t ha⁻¹). Canopy leaf area was also significantly larger in mixed stands (1.96m² m⁻²) than in oak stands (1.73m² m⁻²). Soil organic carbon (C _org) and moisture were greater in mixed fir-beech stands, probably as a result of increased cover. Soil microbial biomass carbon (C _mic), nitrogen (N _mic), and total soil nitrogen (N _tot) increased slightly in the mixed stand, although this difference was not significant. Overall, mixed stands showed a higher mean C _org/N _tot ratio (22.73) compared to oak stands (16.39), indicating relatively low rate of carbon mineralization. In addition, the percentage of organic C present as C _mic in the surface soil decreased from 3.17% in the oak stand to 2.26% in the mixed stand, suggesting that fir-beech litter may be less suitable as a microbial substrate than oak litter.     

Soil compaction and poultry litter effects on factors affecting nitrogen availability in a claypan soil

Soil compaction may affect N mineralization and the subsequent fate of N in agroecosystems. Laboratory incubation and field experiments were conducted to determine the effects of surface soil compaction on soil N mineralization in a claypan soil amended with poultry litter (i.e., Turkey excrement mixed with pine shavings as bedding). In a laboratory study, soil from the surface horizon of a Mexico silt loam soil was compacted to four bulk density levels (1.2, 1.4, 1.6 and 1.8 Mg m⁻³) with and without poultry litter and incubated at 25 °C for 42 days. A field trial planted to corn (Zea mays L.) was also conducted in 2002 on a Mexico silt loam claypan soil in North Central Missouri. Soil was amended with litter (0 and 19 Mg ha⁻¹) and left uncompacted or uniformly compacted. Soil compaction decreased soil inorganic N by a maximum of 1.8 times in the laboratory study; this effect was also observed at all depths of the field trial. Compacted soil with a litter amendment accumulated NH₄⁺-N up to 7.2 times higher than the noncompacted, litter-amended soil until Day 28 of the laboratory incubation and in the beginning of the growing season of the field study. Ammonium accumulation may have been due to decreased soil aeration under compacted conditions. Application of litter increased soil N mineralization throughout the growing season. In the laboratory study, soil inorganic N in unamended soil was negatively correlated with soil bulk density and the proportion of soil micropores, but was positively related with soil total porosity and the proportion of soil macropores. These results indicate that soil compaction, litter application and climate are interrelated in their influences on soil N mineralization in agroecosystems.     

Arsenic accumulation by rice grown in soil treated with roxarsone

Poultry litter is widely used as a fertilizer for lowland rice in Taiwan and China. However, the organic‐arsenic compound roxarsone (additive of poultry feed) in poultry litter can be absorbed by the plants and the resulting arsenic (As) contamination may pose a serious threat to human health. This study used various amounts of poultry litter contaminated with roxarsone in pot experiments to evaluate the effect of roxarsone on rice agronomic parameters and the bioaccumulation of total and inorganic As in rice‐plant tissues. Rice‐grain yield decreased significantly with increasing As content of the soil, and the critical threshold that killed rice was 200 mg roxarsone (kg soil)^–1. The As concentrations in root, straw, leaf, husk, and grain increased with increasing soil As (p < 1%). At 100 mg roxarsone per kg of soil, the As concentration in the rice grain exceeded the statutory permissible limit of 1.0 mg As (kg dry weight)^–1 and at 25 mg roxarsone (kg soil)^–1, the inorganic As concentrations in grains exceeded the statutory limit of 0.15 mg of inorganic As kg^–1 in China. For all treatments, the As concentrations in various plant tissues at maturity follow the order: root > stem > leaf > husk > grain. Arsenite was the predominant species in root, straw, and grain, while arsenate was the predominant species in leaf and husk. No significant difference existed between the amounts of arsenite and arsenate when various amounts of poultry litter were applied. This result illustrates that large amounts of added roxarsone are not only toxic to rice but also accumulate in grains in the inorganic As forms, potentially posing a threat to human health via the food chain.     

Ionic Activity in Soil Solution as Affected by Application Of Newsprint and Nitrogen Sources

The influence of noncomposted ground newsprint (GNP) and nitrogen (N) source on corn (Zea Mays L.) dry matter production, grain yield, and soil chemical properties has been previously reported (Lu et al. 1995). The effects of GNP and N source on soil solution ionic activities at 40 days after planting in a field study; seed germination and extractable aluminum (Al) in GNP in laboratory studies were evaluated to determine their effects on corn seedling stunting and nutrient imbalances during early growth stages. Ammonium nitrate (NH₄NO₃), urea, anhydrous ammonia (NH₃), or poultry litter (PL) were the N sources used in the field study to adjust the C:N ratio of the GNP to ≤ 30:1. In laboratory experiments, cotton, soybean, and corn germination at seven days was not affected by N source or GNP applied at a rate of 2.44 kg C/m² soil; but N source did influence the dry weight of corn root/shoot ratio at 21 days. The Al extracted from GNP increased as the NH₄OH concentration in the extracting solution increased and followed a quadratic relationship with an r² of 0.90. The σα_ca/σα_cation ratio in soil solution for all N sources was greater than the 0.15 reported by Bennett and Adams (1970a), where incipient NH₃ phytotoxicity can occur. At 40 days after planting, a two-fold increase in soil monomeric Al (σAlα_mon.) ionic activity and a five-fold increase in soil P (σPα) ionic activity were measured in GNP treatments as compared to no N GNP treatments. When NH₃ was the N source used to adjust the C:N ratio of GNP, the σAlα_mon. ionic activities were increased by a factor of five as compared to NH₃ applied alone. When PL was the N source, the Al_T (σAlα_species) ionic activity was 119 mmol L^?1 compared to the σAlα_mon. ionic activity of 0.53 mmol L^?1. It appears that σAlα_mon. ionic activity induced nutrient disorder and caused severe stunting of corn seedlings during early season growth. The relatively high water-soluble organic carbon in PL (18 percent) may have acted as a chelating agent to reduce the σAlα_mon. ionic activities in the GNP, or as a soluble carbon source for increasing microbial utilization of all the N, thus slowing the formation and accumulation of phytotoxic levels of by-products.     

Phosphorus fractions of soils under <Emphasis Type="Italic">Lotus corniculatus</Emphasis> as affected by different phosphorus fertilizers

The objective of this work was to assess the changes of soil P fractions by Lotus corniculatus and to determine contribution of each fraction to plant P nutrition. Phosphorus was added at a rate of 240 mg/pot as triple superphosphate (20% P), phosphate rock (13% P), or poultry litter (2% P) to a Vertisol or an Inceptisol; a control treatment (without P fertilizer) was also included. Then, L. corniculatus was sowed and harvested eight times; both yields and P content of plant were determined at each harvest. Soil P fractions were determined by Hedley’s modified method. The content of labile and moderately labile P [anionic exchange membrane-Pi (AEM-Pi), NaHCO₃-Pi, and NaOH-Pi] fractions were markedly reduced and were probably due to P uptake by plants. The content of the HCl-Pi fraction of the phosphate-rock-treated soil decreased whereas that of the residual P fraction was not modified. The content of organic forms increased in all treatments. The content of both labile organic P and moderately labile organic P were positively and significantly correlated with the P concentration of roots and with roots biomass, suggesting that the increase in these two organic fractions was related to root production. AEM-Pi accounted for 95% and 84% of absorbed P in Vertisol and Inceptisol, respectively.     

Soil organic matter, microbial biomass and enzyme activities in a tropical agroforestry system

 The effects of growing trees in combination with field crops on soil organic matter, microbial biomass C, basal respiration and dehydrogenase and alkaline phosphatase activities were studied in soils under a 12-year-old Dalbergia sissoo (a N₂-fixing tree) plantation intercropped with a wheat (Triticum aestivum) – cowpea (Vigna sinensis) cropping sequence. The inputs of organic matter through D. sissoo leaf litter increased and crop roots decreased with the increase in tree density. Higher organic C and total N, microbial biomass C, basal soil respiration and activities of dehydrogenase and alkaline phosphatase were observed in treatments with tree-crop combination than in the treatment without trees. Soil organic matter, microbial biomass C and soil enzyme activities increased with the decrease in the spacing of the D. sissoo plantation. The results indicate that adoption of the agroforestry practices led to an improved organic matter status of the soil, which is also reflected in the increased nutrient pool and microbial activities necessary for long-term productivity of the soil. However, tree spacing should be properly maintained to minimize the effects of shading on the intercrops. Received: 21 February 1997     

Effect of density and species richness of soil mesofauna on nutrient mineralisation and plant growth

The aim of this study was to test the relative importance of changes in density and species richness of soil mesofauna as determinants of nutrient mineralisation and plant growth. The experiment was carried out using microcosms containing a mixture of plant litter and soil in which seedlings of Lolium perenne were planted, and a range of combinations of levels of density and species richness of microarthropods added. Over the duration of the experiment, nutrient release, measured as concentrations of NO₃ ^--N and total N in leachates, increased significantly with increasing microarthropod density, but decreased with increasing species richness. Leachate concentrations of NH₄ ⁺-N, dissolved organic N and C (DON and DOC) were not affected by the faunal treatments. Soil respiration, a measure of microbial activity, decreased with increasing density of microarthropods, whereas microbial biomass was not affected by microarthropods. Increasing density of soil animals had a negative effect on the shoot biomass of L. perenne while the effect of species richness was positive. Neither the species richness nor density of soil microarthropods was found to significantly influence root biomass. We conclude that variations in animal density had a greater influence on soil nutrient mineralisation processes than did species richness. Possible reasons for these opposing effects of animal density and diversity on soil N mobilization are discussed.     

Effect of plant growth-promoting bacteria and soil compaction on barley seedling growth, nutrient uptake, soil properties and rhizosphere microflora

Inoculants are of great importance in sustainable and/or organic agriculture. In the present study, plant growth of barley (Hordeum vulgare) has been studied in sterile soil inoculated with four plant growth-promoting bacteria and mineral fertilizers at three different soil bulk densities and in three harvests of plants. Three bacterial species were isolated from the rhizosphere of barley and wheat. These bacteria fixed N₂, dissolved P and significantly increased growth of barley seedlings. Available phosphate in soil was significantly increased by seed inoculation of Bacillus M-13 and Bacillus RC01. Total culturable bacteria, fungi and P-solubilizing bacteria count increased with time. Data suggest that seed inoculation of barley with Bacillus RC01, Bacillus RC02, Bacillus RC03 and Bacillus M-13 increased root weight by 16.7, 12.5, 8.9 and 12.5% as compared to the control (without bacteria inoculation and mineral fertilizers) and shoot weight by 34.7, 34.7, 28.6 and 32.7%, respectively. Bacterial inoculation gave increases of 20.3–25.7% over the control as compared with 18.9 and 35.1% total biomass weight increases by P and NP application. The concentration of N and P in soil was decreased by increasing soil compaction. In contrast to macronutrients, the concentration of Fe, Cu and Mn was lower in plants grown in the loosest soil. Soil compaction induced a limitation in root and shoot growth that was reflected by a decrease in the microbial population and activity. Our results show that bacterial population was stimulated by the decrease in soil bulk density. The results suggest that the N₂-fixing and P-solubilizing bacterial strains tested have a potential on plant growth activity of barley.     

Phosphatase activities in soil after repeated untreated and alum-treated poultry litter applications

Repeated additions of untreated and aluminum sulfate (alum)-treated poultry litter to soil affect ecology and consequent nutrient dynamics. The objective of this study was to determine how repeated annual poultry litter additions affected phosphatase activities in concert with changes in soil phosphorus (P). Field plots were amended annually since 1995 with either 2.24 or 8.96 Mg ha⁻¹ alum-treated (AL-1 or AL-4, respectively) or untreated poultry litter (PL-1 or PL-4, respectively) or equivalent rates by N content of ammonium nitrate (AN-1 or AN-4, respectively). Soil pH, total C (C_Tot), microbial biomass C, double-stranded deoxyribonucleic acid (dsDNA), Mehlich-III P (M3P), water-soluble P (WSP), and acid and alkaline phosphatase activities were measured before and 10 days, 1 month, and 6 months after fertilizer applications in 2003 and 2004. M3P and acid phosphatase activities were higher in AL-4 soil than in all other treatments. Higher WSP in the untreated compared to the alum-treated litter resulted in higher WSP in the soil amended with untreated litter. At the same time, alkaline phosphatase activities were significantly higher in PL-4 compared to AL-4. In contrast, alkaline phosphatase activities were significantly lower in AN-4, the treatment with the lowest pH. Additionally, alkaline phosphatase activities expressed per unit C_Tot or dsDNA remained significantly greater in PL-4 and significantly lower in AN-4, than all other treatments. Thus, some factor beyond an increase in soil C or microbial biomass contributed to elevated alkaline phosphatase activities in PL-4, despite increased WSP in the treatment receiving the high rate of untreated litter.     

Soil microbial activity and litter turnover in native grazed and ungrazed rangelands in a semiarid ecosystem

Long-term overgrazing is known to influence soil microbiological properties and C sequestration in soil organic matter. However, much remains to be known concerning overgrazing impacts on soil microbial activity and litter turnover in heavily grazed rangelands of Central Iran. Aboveground litter decomposition of three dominant species (Agropyron intermedium, Hordeum bulbosum, and Juncus sp.) were studied using a litter bag experiment under field conditions in three range sites of Central Iran, a site with continuous grazing, a site ungrazed for 17 years with dominant woody species (80% cover), and a site ungrazed for 17 years with dominant pasture species (70% cover). Soil samples were taken from 0 to 30 cm depth and analyzed for their chemical and microbiological properties. Results demonstrate that soil organic C and total N contents and C/N ratios were similar for both ungrazed and grazed sites, while available P and K concentrations significantly decreased under grazed conditions. It was also evident that range grazing decreases soil respiration and microbial biomass C, suggesting a lower recent annual input of decomposable organic C. Nevertheless, grazing conditions had no significant effect on litter decomposition indicating soil microclimate is not affected by grazing animals in this ecosystem. It is concluded that overgrazing may presumably depress microbial activity through either reduced input of fresh plant residue into the surface soil or lack of living roots and exudates for stimulating microbial activity. This study also suggests that 17 years of livestock exclusion might be insufficient time for expected C accumulation in soil.     

西南山地林下经济模式对土壤养分和土壤微生物数量的影响

为进一步加强农林牧业资源共享,推动生态农业的发展,在重庆市荣昌县6种不同的林下经济模式下,分别对土壤养分和微生物数量的变化及其相互关系进行了研究。结果表明,不同的林下经济模式对土壤养分的影响各有差异,有机质和全钾在麻竹林下养禽模式中增加最多,全磷在桉树林下养禽模式中增加最多,全氮、碱解氮、速效钾含量则在马尾松林下养畜模式中增加最多。从微生物总量来看,土壤中表现为桉树林下禽模式最多,其次为桉树林下种菌模式,最少的是麻竹林下种菌模式;腐殖层中最多的是麻竹林下养禽模式,最少的是桉树林下种菌模式。微生物数量与有机质、有效磷和碱解氮呈正相关关系。由于土壤养分和微生物数量的变化趋势不一致,因此在研究林下经济过程中应将二者结合起来。     

Nine years of enriched CO2 changes the function and structural diversity of soil microorganisms in a grassland

To gain insight into microbial function following increased atmospheric CO₂ concentration, we investigated the influence of 9 years of enriched CO₂ (600 μl litre⁻¹) on the function and structural diversity of soil microorganisms in a grassland ecosystem under free air carbon dioxide enrichment (FACE), as affected by plant species (Trifolium repens L. and Lolium perenne L. in monocultures and mixed culture) and nitrogen (N) supply. We measured biomass and activities of enzymes covering cycles of the most important elements (C, N and P). The microbial community was profiled by molecular techniques of phospholipid fatty acid (PLFA) and denaturing gradient gel electrophoresis (DGGE) analysis. The enrichment in CO₂ increased soil microbial biomass (+48.1%) as well as activities of invertase (+36.2%), xylanase (+22.9%), urease (+23.8%), protease (+40.2%) and alkaline phosphomonoesterase (+54.1%) in spring 2002. In autumn, the stimulation of microbial biomass was 25% less and that of enzymes 3–12% less than in spring. Strong correlations between activities of invertase, protease, urease and alkaline phosphomonoesterase and microbial biomass were found. The stimulation of microbial activity in the enriched atmosphere was probably caused by changes in the quantity and kind of root litter and rhizodeposition. The response of soil microorganisms to enriched CO₂ was most pronounced under Trifolium monoculture and under greater N supply. The PLFA analysis revealed that total PLFA contents were greater by 24.7% on average, whereby the proportion of bioindicators representative of Gram‐negative bacteria increased significantly in the enriched CO₂ under less N‐fertilized Lolium culture. Discriminant analysis showed marked differences between the PLFA profiles of the three plant communities. Shannon diversity indices calculated from DGGE patterns were greater (+12.5%) in the enriched CO₂, indicating increased soil bacterial diversity. We conclude that greater microbial biomass and enzyme activity buffer the potential increase in C sequestration occurring from greater C addition in enriched CO₂ due to greater mineralization of soil organic matter.     

Nutrient accumulation and nitrate leaching under broiler litter amended corn fields

Abstract

Alabama's broiler chicken (Gallus gallus) industry produces large amounts of waste, which are disposed of by application to crop and pasture land. Land application of litter (manure and bedding) from broiler production can lead to contamination from losses of nutrients accumulated in soil. A study was conducted on 2 and 4% slopes from 1991 to 1993 at Belle Mina, Alabama, to determine the effects of broiler litter (BL) on soil elemental concentrations and nitrate leaching under a corn (Zea mays L.) ‐ winter rye (Secale cereale L.) cropping system amended with either: l) 9 mg#lbha^‐1 of BL, 2) 18 mg#lbha^‐1 of BL, or 3) commercial fertilizer (F) at a recommended rate. Soil was sampled to 100 cm prior to corn planting and subsequent to com harvest. Soil leachate samples were collected biweekly with wick lysimeters installed at a depth of 100 cm. Litter applications increased concentrations of soil organic carbon (C), extractable phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), copper (Cu) and zinc (Zn). Post harvest soil sampling indicated leaching of soil nitrate that was generally highest under BL18. Soil electrical conductivity measurements were highest under BL18, but values were not in the range considered detrimental to crops. Nitrate‐N (NO₃‐N) concentrations measured in soil percolate at 1‐m depth on the 2% slope were higher under F than litter treatments. Both the F and BL18 treatments produced some NO₃‐N concentrations above the primary drinking water standard, but averaged only 8.3 and 4.8 mg#lbL^‐1, respectively. The BL9 treatment consistently remained under 10 mg NO₃‐N#lbL^‐1 with a mean concentration of 1.3 mg#lbL^‐1. Overall, litter applied a 9 mg#lbha^‐1 produced agronomic results comparable to F and appeared to be the optimal rate of application under the conditions of this study.     

Plant species richness leaves a legacy of enhanced root litter-induced decomposition in soil

Increasing plant species richness generally enhances plant biomass production, which may enhance accumulation of carbon (C) in soil. However, the net change in soil C also depends on the effect of plant diversity on C loss through decomposition of organic matter. Plant diversity can affect organic matter decomposition via changes in litter species diversity and composition, and via alteration of abiotic and/or biotic attributes of the soil (soil legacy effect). Previous studies examined the two effects on decomposition rates separately, and do therefore not elucidate the relative importance of the two effects, and their potential interaction. Here we separated the effects of litter mixing and litter identity from the soil legacy effect by conducting a factorial laboratory experiment where two fresh single root litters and their mixture were mixed with soils previously cultivated with single plant species or mixtures of two or four species. We found no evidence for litter-mixing effects. In contrast, root litter-induced CO₂ production was greater in soils from high diversity plots than in soils from monocultures, regardless of the type of root litter added. Soil microbial PLFA biomass and composition at the onset of the experiment was unaffected by plant species richness, whereas soil potential nitrogen (N) mineralization rate increased with plant species richness. Our results indicate that the soil legacy effect may be explained by changes in soil N availability. There was no effect of plant species richness on decomposition of a recalcitrant substrate (compost). This suggests that the soil legacy effect predominantly acted on the decomposition of labile organic matter. We thus demonstrated that plant species richness enhances root litter-induced soil respiration via a soil legacy effect but not via a litter-mixing effect. This implies that the positive impacts of species richness on soil C sequestration may be weakened by accelerated organic matter decomposition.     

竹生物质炭和竹凋落物对毛竹林土壤营养元素和酶活性的影响

以广西壮族自治区桂林市华江乡内广泛分布的毛竹林土壤为研究对象,以竹生物质炭和竹凋落物作为外源碳,设置对照（CK）、低添加量生物质炭（1% BC）、高添加量生物质炭（2% BC）、低添加量凋落物（1% L）、高添加量凋落物（2% L）5个处理,进行为期两个月的室内培养试验,研究不同外源碳添加对毛竹林土壤营养元素和酶活性的影响。结果表明：与对照相比,竹生物质炭和竹凋落物添加均显著提高了土壤pH;竹生物质炭添加显著降低了而竹凋落物添加显著提高了土壤铵态氮（NH₄⁺-N）含量（P<0.05）,且高添加量（2% BC和2% L）的降低或提高作用更明显;不同外源碳添加均显著提高了土壤硝态氮（NO₃^–-N）含量,且凋落物添加的提高作用更明显;不同外源碳添加均显著提高了土壤有效磷（AP）含量,且高添加量的提高作用更明显;竹生物质炭添加对土壤可溶性有机碳（DOC）含量没有显著影响,但降低了土壤可溶性氮（DN）含量,而竹凋落物添加显著提高了土壤DOC和DN含量;不同外源碳添加对土壤微生物生物量碳（MBC）和氮含量（MBN）均没有显著影响,但降低了土壤蔗糖酶和脲酶活性。相关性分析表明,土壤pH、NH₄⁺-N、NO₃^–-N、DOC和DN是影响竹林土壤酶活性的关键性因子。     

凋落物输入改变对慈竹林土壤有机碳的影响

通过野外试验,研究了凋落物输入改变对慈竹林土壤有机碳的影响。设置10%,50%和70%这3种去除凋落物量和分别添加15%,25%的绵竹、杉木凋落物处理及对照。结果发现,去除凋落后慈竹林土壤有机碳含量明显减少(p<0.05),而且去除量越大,有机碳含量减少越大。添加外源凋落物显著提高了慈竹林土壤有机碳含量(p<0.05),其中,15%的添加比例比25%更有利于土壤有机碳的积累,添加绵竹凋落物对土壤有机碳增加的效应高于添加杉木凋落物。研究表明,选择适合的树种和恰当的混交比例以及加强对凋落物层的保护,对维持慈竹林土壤有机碳具有重要的意义。