Impeded drainage stimulates extracellular phenol oxidase activity in riparian peat cores

Peat drainage, a common land‐use practice in Europe, has been associated with habitat degradation and increased particulate and dissolved carbon release. In the UK, peatland drain blockage has been encouraged in recent years as a management practice to preserve peatland habitats and to reduce fluvial carbon loss and municipal water discoloration. Drain blockage has, however, been found to increase drain‐water dissolved organic carbon (DOC) concentrations and coloration in the short term. In order to investigate the contribution of changes in extracellular phenol oxidase activity to the increase in water coloration following peatland drain blockage, cores collected from a riparian peatland in North Wales were incubated under impeded drainage conditions. Impeded drainage resulted in the stimulation of peat extracellular phenol oxidase activity and heightened soluble phenolic concentrations, suggesting that changes in extracellular phenol oxidase activity may be a key driver of increases in DOC and water coloration following peatland drain blockage. An increase in peat pH with impeded drainage was also observed that may have contributed to the heightened soluble phenolic concentrations – directly (through effects on solubility) and/or indirectly as a driver of the elevated extracellular phenol oxidase activity.     

Long‐term field fertilization affects soil nitrogen transformations in a rice‐wheat‐rotation cropping system

Mineralization and nitrification are the key processes of the global N cycle and are primarily driven by microorganisms. However, it remains largely unknown about the consequence of intensified agricultural activity on microbial N transformation in agricultural soils. In this study, the ¹⁵N‐dilution technique was carried out to investigate the gross mineralization and nitrification in soils from a long‐term field fertilization experiment starting from 1988. Phospholipid fatty acids (PLFA) analysis was used to determine soil microbial communities, e.g., biomasses of anaerobic bacterial, bacterial, fungi, and actinobacteria. The abundance of ammonia‐oxidizing bacteria (AOB) and archaea (AOA) were measured using real‐time quantitative polymerase chain reaction. The results have demonstrated significant stimulation of gross mineralization in the chemical‐fertilizers treatment (NPK) ([6.53 ± 1.29] mg N kg^–1 d^–1) and chemical fertilizers–plus–straw treatment (NPK+S1) soils ([8.13 ± 1.68] mg N kg^–1 d^–1) but not in chemical fertilizers–plus–two times straw treatment (NPK+S2) soil when compared to the control‐treatment (CK) soil ([3.62 ± 0.86] mg N kg^–1 d^–1). The increase of anaerobic bacterial biomass is up to 6‐fold in the NPK+S2 compared to that in the CK soil ([0.7 ± 0.5] nmol g^–1), implying that exceptionally high abundance of anaerobic bacteria may inhibit gross mineralization to some extent. The gross nitrification shows upward trends in the NPK+S1 and NPK+S2 soils. However, it is only significantly higher in the NPK soil ([5.56 ± 0.51] mg N kg^–1 d^–1) compared to that in the CK soil ([3.70 ± 0.47] mg N kg^–1 d^–1) (p < 0.05). The AOB abundance increased from (0.28 ± 0.07) × 10⁶ copies (g soil)^–1 for the CK treatment to (4.79 ± 1.23) × 10⁶ copies (g soil)^–1 for the NPK treatment after the 22‐year fertilization. In contrast, the AOA abundance was not significantly different among all treatment soils. The changes of AOB were well paralleled by gross nitrification activity (gross nitrification rate = 0.263 AOB + 0.047 NH ‐N + 2.434, R² = 0.73, p < 0.05), suggesting the predominance of bacterial ammonia oxidation in the fertilized fields.     

Long‐term Effects of Pine Plantations on Soil Quality in Southern Spain

This work evaluated how pine plantations established on old fields and degraded lands influence soil properties in comparison with adjacent unplanted areas that undergo into secondary succession, and native forests, analysing the effects of abiotic variables and stand characteristics in the afforestation process. Thirty‐two paired sites (pine plantations versus unplanted areas) and 10 native forests were selected in the SE Spain. In total, 74 soil profiles were studied, and 222 composite soil samples were collected at three different depths. Soil organic carbon, cation exchange capacity, and C : N ratio showed significantly greater values in pine plantations in relation to the unplanted areas (0–5 cm), and the mean values of soil organic carbon, nitrogen (N), C : N ratio, and cation exchange capacity in these pine plantations were similar to those found under native forests. Only K⁺ concentrations were clearly higher in the native forests than in the other land uses for all depths analysed. Pine plantations in the drier and warmer areas showed lower soil quality in relation to the paired unplanted areas, as well as the younger and denser ones; it may be because under these situations, more time is needed to produce an improvement. In fact, the paired net variations increased with the stand age and/or tree size. In conclusion, pine plantations were in general more efficient in improving parameters related to soil quality, especially in locations with high soil water retention capacity, which in our study area were found at higher and cooler elevations. Copyright © 2016 John Wiley & Sons, Ltd.     

Summer drought effects upon soil and litter extracellular phenol oxidase activity and soluble carbon release in an upland Calluna heathland

Extracellular phenol oxidases play an important role in the soil carbon cycle. The effects of a field-scale summer drought manipulation on extracellular litter and soil phenol oxidase activity, soluble phenolic compounds and dissolved organic carbon concentrations were examined for an upland Calluna heathland on a peaty podsol in North Wales. Litter and organic soil phenol oxidase activity was found to be positively correlated with moisture content. Thus in shallow organic soils, which are sensitive to drying during periods of low rainfall, drought may inhibit soil phenol oxidase activity as a result of water limitations. The release of soluble phenolic compounds and DOC from the droughted plots was found to be lowered during the drought period and elevated outside of the drought period. It is hypothesized that these changes may be a result of the reduced ability of extracellular phenol oxidases to process recalcitrant polyphenolic material under drought conditions. A drying incubation carried out with litter and soil cores from the same site suggests that extracellular phenol oxidase activity displays an optimal moisture level. This reconciled the observed water limitation of phenol oxidase activity at the heathland experimental site with previously observed stimulation of phenol oxidase activity by water table drawdown in deeper peats.     

Long‐term carbon loss and CO2‐C release of drained peatland soils in northeast Germany

Peatlands are common in many parts of the world. Draining and other changes in the use of peatlands increase atmospheric CO₂ concentration. If we are to make reliable quantitative predictions of that effect, we need good information on the CO₂ emission rates from peatlands. The present study uses two different methods for predicting CO₂‐C release of peatland soils: (i) a 40‐year field investigation of balancing organic carbon stocks and (ii) short‐term CO₂‐C release rates from laboratory experiments. To estimate long‐term losses of peat, and its resulting C input to the atmosphere, we combined highly detailed maps of surface topography and its changes, and the organic C contents and bulk densities of a drained peatland from different years. Short‐term CO₂‐C release rates were measured in the laboratory by incubating soil samples from several soil horizons at various temperatures and soil moistures. We then derived nonlinear CO₂‐C production functions, which we incorporated into a numerical simulation model (HYDRUS). Using HYDRUS, we calculated daily soil water components and CO₂‐release for (i) real‐climate data from 1950 to 2003 and (ii) a climate scenario extending to 2050, including an increase in temperature of 2°C and 20% less rainfall during the summer half year, i.e. from April to September inclusive. From our field measurements, we found a mean annual decrease of 0.7 cm in the thickness of the peat. Large losses (> 1.5 cm year^?1) occurred only during periods when groundwater levels were low (i.e. a deep water‐table). The annual CO₂‐C release results in a mean loss from the peat of about 700 g CO₂‐C m^?2, mostly as a direct contribution to the atmosphere. Both methods produced very similar results. The model scenarios demonstrated that CO₂‐C loss is mainly controlled by the groundwater (i.e. water‐table) depth, which controls subsurface aeration. A local climate scenario estimated a c. 5% increase of CO₂‐C losses within the next 50 years.     

Long‐term changes in the floristic composition and soil characteristics of reclaimed sodic land during eco‐restoration

The study was conducted under the “Uttar Pradesh Sodic Lands Reclamation Project” to examine changes that occurred in the reclaimed sodic land in two districts of Uttar Pradesh, India. The study focuses on long‐term seasonal changes in the floral diversity and soil characteristics of the reclaimed sodic land over a period of 10 y. The changes in the floristic composition, plant density, and soil characteristics (microbial biomass carbon [MBC], pH, exchangeable‐sodium percentage (ESP), and electrical conductivity) were compared among the different study plots after different years of sodic‐land reclamation. The study plots comprised reclaimed land with rice–wheat cultivation; semireclaimed land under rice cultivation only and nonreclaimed barren sodic land. There was a significant variation in the floristic composition of the three study plots. Dominance in the floristic composition was shifted from monocotyledonous weeds in the nonreclaimed sodic land to dicotyledonous weeds in the reclaimed land after 10 y of reclamation. Among the soil characteristics, the most remarkable changes were observed in soil MBC and ESP during the course of sodic‐land reclamation. Soil MBC increased up to 480% and ESP values decreased up to 79% in the reclaimed plots with reference to the nonreclaimed plots. The soil amelioration was more pronounced in the upper layer (0–30 cm) as compared to the lower layer (below 30 cm depth). A positive significant correlation was revealed between soil MBC and floristic composition of the reclaimed plots. These changes in floristic composition and soil characteristics could be used as good indicators of the eco‐restoration of the sodic lands. The present study provides useful insights in understanding the temporal progress of eco‐restoration in the reclaimed sodic lands.     

Fifty Years on: Long‐term Patterns of Land Sensitivity to Desertification in Italy

The Mediterranean region has been regarded as a critical hotspot for desertification due to the impact of soil degradation, the land‐use changes and the climate variations. Few large‐scale studies have been devoted to analyse trends in land sensitivity to desertification in the northern Mediterranean basin. The present paper contributes to this deserving issue by quantifying the level of land sensitivity to desertification in Italy at seven points between 1960 and 2010 at a fine spatial scale. The approach used followed the Environmentally Sensitive Area scheme that assesses changes in four key themes (climate, soil, vegetation and land management) related to land degradation processes. Italian land was classified into four levels of sensitivity to desertification (non‐affected, potentially affected, fragile and critical) according to the Environmentally Sensitive Area framework. Interestingly, although land surface area classified as ‘fragile’ and ‘critical’ grew homogeneously in Italy between 1960 and 1990, the increase observed in the most recent time period was spatially clustered and contributed to reverse the polarisation in ‘structurally vulnerable’ and ‘non‐affected’ regions observed in Italy. The paper discussed these trends in the light of socioeconomic changes that occurred in Italy after World War II. Copyright © 2013 John Wiley & Sons, Ltd.     

Nitrogen mineralization from mature bio‐waste compost in vineyard soils. I. Long‐term laboratory incubation experiments

The steadily increasing utilization of bio‐waste compost in German viticulture requires a more detailed investigation of nitrogen (N) mineralization parameters for mature bio‐waste compost applied to vineyard soils. N mineralization kinetics were described with two superposing exponential equations. Long‐term aerobic laboratory incubation experiments of 12 soil‐compost substrates revealed that 5±2.8% of its total N content could be released from a rapidly decomposable fraction (half‐life period t₅₀ = 41 d at 15°C) and another 60±2.9% from a slower decomposable fraction (t₅₀ = 490 d). The remaining proportion (35%) is considered not to be released in the medium term. The obtained potentially mineralizable nitrogen of 65% of total compost N significantly differs from current fertilizer recommendations, which were adopted from calculations for agricultural conditions. For fertilizer recommendations in viticulture, we recommend the consideration of a higher N‐mineralization potential for organic fertilizers.     

Long‐term effects of fertilization on some soil properties under rainfed soybean‐wheat cropping in the Indian Himalayas

This study aims to examine the effects of long‐term fertilization and cropping on some chemical and microbiological properties of the soil in a 32 y old long‐term fertility experiment at Almora (Himalayan region, India) under rainfed soybean‐wheat rotation. Continuous annual application of recommended doses of chemical fertilizer and 10 Mg ha^–1 FYM on fresh‐weight basis (NPK + FYM) to soybean (Glycine max L.) sustained not only higher productivity of soybean and residual wheat (Triticum aestivum L.) crop, but also resulted in build‐up of total soil organic C (SOC), total soil N, P, and K. Concentration of SOC increased by 40% and 70% in the NPK + FYM–treated plots as compared to NPK (43.1 Mg C ha^–1) and unfertilized control plots (35.5 Mg C ha^–1), respectively. Average annual contribution of C input from soybean was 29% and that from wheat was 24% of the harvestable aboveground biomass yield. Annual gross C input and annual rate of total SOC enrichment from initial soil in the 0–15 cm layer were 4362 and 333 kg C ha^–1, respectively, for the plots under NPK + FYM. It was observed that the soils under the unfertilized control, NK and N + FYM treatments, suffered a net annual loss of 5.1, 5.2, and 15.8 kg P ha^–1, respectively, whereas the soils under NP, NPK, and NPK + FYM had net annual gains of 25.3, 18.8, and 16.4 kg P ha^–1, respectively. There was net negative K balance in all the treatments ranging from 6.9 kg ha^–1 y^–1 in NK to 82.4 kg ha^–1 y^–1 in N + FYM–treated plots. The application of NPK + FYM also recorded the highest levels of soil microbial‐biomass C, soil microbial‐biomass N, populations of viable and culturable soil microbes.     

Long‐term tillage effects on the distribution of phosphorus fractions of loess soils in Germany

Different tillage systems may affect P dynamics in soils due to differently distributed plant residues, different aggregate dynamics and erosion losses, but quantitative data are scarce. Objectives were to investigate the effect of tillage on the availability of P in a long‐term field trial on loess soils (Phaeozems and Luvisols) initiated from 1990 to 1997. Four research sites in E and S Germany were established with a crop rotation consisting of two times winter wheat followed by sugar beet. The treatments were no‐till (NT) without cultivation, except for seedbed preparation to a depth of 5 cm before sugar beet was sown and conventional tillage (CT) with mouldboard plowing down to 25–30 cm. Soil P was divided into different pools by a sequential extraction method, and total P (P_t) in the single P fractions was extracted by digesting the extracts of the fractionation to calculate the contents of organic P. The P_t content (792 mg [kg soil]^–1) in the topsoil (0–5 cm) of NT was 15% higher compared to CT, while with increasing depth the P_t content decreased more under NT than under CT. This was also true for the other P fractions except for residual P. The higher P contents in the topsoil of NT presumably resulted from the shallower incorporation of harvest residues and fertilizer P compared to CT, whereas estimated soil losses and thus also P losses due to water erosion were only small for both treatments. Contents of oxalate‐extractable Fe and organic C were positively related to the labile fractions of inorganic P, while there was a high correlation of the stable fractions with the clay contents and pH. Multiple regression analyses explained 50% of the variability of these P fractions. Overall, only small differences in the P fractions and availability were observed between the long‐term tillage treatments.     

Long‐term fertilization impacts on corn yields and soil organic matter on a clay‐loam soil in Northeast China

A long‐term fertilization experiment with monoculture corn (Zea mays L.) was established in 1980 on a clay‐loam soil (Black Soil in Chinese Soil Classification and Typic Halpudoll in USDA Soil Taxonomy) at Gongzhuling, Jilin Province, China. The experiment aimed to study the sustainability of grain‐corn production on this soil type with eight different nitrogen (N)‐, phosphorus (P)‐, and potassium (K)–mineral fertilizer combinations and three levels (0, 30, and 60 Mg ha^–1 y^–1) of farmyard manure (FYM). On average, FYM additions produced higher grain yields (7.78 and 8.03 Mg ha^–1) compared to the FYM0 (no farmyard application) treatments (5.67 Mg ha^–1). The application of N fertilizer (solely or in various combinations with P and K) in the FYM0 treatment resulted in substantial grain‐yield increases compared to the FYM0 control treatment (3.56 Mg ha^–1). However, the use of NP or NK did not yield in any significant additional effect on the corn yield compared to the use of N alone. The treatments involving P, K, and PK fertilizers resulted in an average 24% increase in yield over the FYM0 control. Over all FYM treatments, the effect of fertilization on corn yield was NPK > NP = NK = N > PK = P > K = control. Farmyard‐manure additions for 25 y increased soil organic‐matter (SOM) content by 3.8 g kg^–1 (13.6%) in the FYM1 treatments and by 7.8 g kg^–1 (27.8%) in the FYM2 treatments, compared to a 3.2 g kg^–1 decrease (11.4%) in the FYM0 treatments. Overall, the results suggest that mineral fertilizers can maintain high yields, but a combination of mineral fertilizers plus farmyard manure are needed to enhance soil organic‐matter levels in this soil type.     

Profile Distribution of Carbon Fractions Under Long‐term Rice‐wheat and Maize‐wheat Production in Alfisols and Inceptisols of Northwest India

The knowledge of profile distribution of soil organic carbon (SOC) in long‐term agricultural systems could help to store atmospheric carbon in the soil. We investigated profile distribution of easily oxidisable Walkley–Black SOC pool (SOC_WB) under long‐term rice‐wheat (R‐W) and maize‐wheat (M‐W) cropping systems under soils of different pedogenesis. The soil samples were collected from the characteristic genetic horizons and analysed for carbon fractions. The SOC_WB was the highest in soils under R‐W systems in both Alfisols and Inceptisols. The SOC_WB stock in the deeper profile horizons under R‐W system was significantly (p < 0·05) higher than that under M‐W system especially in Typic Hapludalfs. Long‐term R‐W system could store on average 3·55 Mg ha⁻¹ more SOC_WB than M‐W system in the Ap horizon. The SOC_WB stock in the Ap horizon of all pedons was significantly (p < 0·05) higher in Alfisols than that in Inceptisols. About 60–90% of the total profile SOC_WB stock was contributed by B‐horizon because of its greater extent. Considering the whole profile, clay was negatively correlated with SOC fractions; however, the SOC fractions were closely related to each other. This study reveals that the distribution of SOC_WB is different in long‐term R‐W and M‐W systems not only in surface but also in the deeper horizons and the magnitude of the variation is influenced by the specific pedogenic processes. This indicates the significance of profile SOC_WB stock instead of topsoil SOC_WB stock in quantifying carbon retention potential of the long‐term management practices. Copyright © 2014 John Wiley & Sons, Ltd.     

Long‐term effects of fertilizer and manure applications on soil quality and yields in a sub‐humid tropical rice‐rice system

Widespread yield stagnation and productivity declines in the rice–rice cropping system have been reported and many of the associated issues are related to soil quality. A long‐term experimental study was initiated in 1969 to assess the impact of continuous cultivation of rice as a single crop grown in wet as well as dry seasons using varying levels of chemical fertilizer and manure applications on soil quality indicators (physical, chemical and biological), a sustainable yield index (SYI) and a soil quality index (SQI). The treatments comprised chemical fertilizers and farmyard manure (FYM) either alone or in combination viz. control, N, NP, NK, NPK, FYM, N+FYM, NP+FYM, NK+FYM and NPK+FYM, laid out in a randomized complete block design with three replications. Soil samples were collected after the wet season rice harvest in 2010 and were analysed for physical, chemical and biological indicators of soil quality. A SYI based on long‐term yield data and SQI using principal component analysis (PCA) and nonlinear scoring functions were calculated. Application of NPK fertilizers in combination with FYM significantly increased the average grain yield of rice in both wet and dry seasons and enhanced the sustainability of the system compared to the control and plots in receipt of fertilizers. The SYI for the control was higher in the wet season than in the dry one, whereas the reverse was true for NPK+FYM treatment. The value of the dimensionless SQI varied from 1.46 in the control plot to 3.78 in the NPK+FYM one. A greater SYI and SQI in the NPK+FYM treatment demonstrated the importance of using a chemical fertilizer in combination with FYM. For the six soil quality indicators selected as a minimum data set (MDS), the contribution of DTPA‐Zn, available‐N and soil organic carbon to the SQI was substantial ranging from 59.4 to 85.7 per cent in NPK+FYM and control plots, respectively. Thus, these soil parameters could be used to monitor soil quality in a subhumid tropical rice–rice system.     

Pollutants in drainage channels following long‐term application of Mancozeb to banana plantations in southeastern Mexico

The aim of this study was to investigate how long‐term Mancozeb application to banana plantations affects the occurrence of pollutants in drainage‐channel sediment and water under tropical conditions. We estimated the possible accumulation of Mancozeb's principal metabolite ethylenethiourea (ETU), as well as manganese (Mn) and zinc (Zn) as components in channel sediment and water. We took samples during the tropical‐rainfall season and the low‐rainfall season. For sediment samples, we determined the contents of ETU, Mn, and Zn. For water samples, we determined the concentration of ETU. Additionally, we took water samples from a runnel that is the receiving body of hydraulic flow from the system. In both seasons, ETU in the sediment was near the detection limit (0.01 mg kg^–1) and did not show any accumulation. However, Mn in sediment at all sampling sites exceeded the threshold values for aquatic life of 630 mg kg^–1 with values between 635 and 7256 mg kg^–1. The Zn concentrations in sediment varied from 87 to 190 mg kg^–1 and exceeded the threshold values for aquatic life of 98 mg kg^–1 at several sites. Furthermore, we determined an accumulation of these heavy metals in the sediments of the banana‐planted zone in comparison with sediments in pasture reference sites. In contrast to the low concentration of ETU in sediment, its concentration in drainage and runnel water (5.9–13.8 μg L^–1) exceeded the EU threshold value for drinking water (0.1 μg L^–1) by up to nearly 140 times. However, the threshold value for aquatic life was not exceeded. We conclude that long‐term Mancozeb application does lead to a severe accumulation of Mn in sediments and of ETU in surface water. New strategies should be used to control black Sigatoka, including integrated methods of pest control so that long‐term negative effects on the environment can be avoided.     

Long‐term effect of compost and inorganic fertilizer on activities of carbon‐cycle enzymes in aggregates of an intensively cultivated sandy loam

The activities of carbon‐cycle enzymes were measured in soil and aggregates to understand compost and inorganic fertilizer amendment effects on soil organic carbon accumulation in an intensively cultivated upland field. Soil samples were collected from a long‐term field experiment with seven treatments: compost, half‐compost N plus half‐fertilizer N, fertilizer NPK, fertilizer NP, fertilizer NK, fertilizer PK and no fertilizer control. The 18‐yr continuous application of compost increased organic C content in soil and three aggregate sizes by 72–124 and 78–234%, respectively, compared with the control. Fertilization also significantly increased organic C contents in soil, macroaggregates and the silt + clay fraction, but not in microaggregates. Compost application significantly reduced the specific activities of polyphenol oxidase (activity per unit organic C) in soil and three aggregate sizes compared with control, whereas fertilization had a much weaker effect. Compost amendment also significantly lowered the specific activities of invertase in macroaggregates and the silt + clay fraction, and this effect was more pronounced than the addition of fertilizer NPK. In contrast, inorganic fertilizer and compost application significantly increased the specific activities of cellobiohydrolase in soil, macroaggregates and microaggregates (but not in the silt + clay fraction), and xylosidase in microaggregates. The application of fertilizer NPK had a more pronounced effect than compost. We considered that the increase in organic C in compost‐amended soil was therefore probably associated with the accumulation of lignocellulose and sucrose in macroaggregates, lignocellulose and hemicellulose in microaggregates and lignin (its derivative) and nonstructural carbohydrates in the silt + clay fraction. However, the application of fertilizer NPK enhanced organic C probably due to an increase in the content of lignin (its derivative) and sucrose in macroaggregates and the silt + clay fraction. Therefore, the application of compost with high lignocellulose should be effective to increase soil organic C in the North China Plain.     

Long‐term effects of lime and gypsum additions on no‐till corn and soybean yield and soil chemical properties in southern Brazil

A long‐term experiment on a clayey, kaolinitic, thermic Rhodic Hapludox where dolomitic lime was applied to the surface (either at 4.5 t/ha or at 1.5 t/ha per yr for 3 yr), or incorporated into the topsoil (4.5 t/ha), and gypsum applied to the surface (3, 6, and 9 t/ha), was carried out to evaluate their effects on soil profile chemical properties and yields of corn (Zea mays L.) and soybean (Glycine max L. Merrill). Lime applied to the soil surface at either full or split rates, or incorporated and surface‐applied gypsum had long‐lasting effects on soil acidity or calcium and sulphur availability respectively, as measured 8 yr after application. Grain yields of corn and soybean were not influenced by liming. Gypsum at 9 t/ha significantly increased corn grain yields by 7 and 8% respectively 7–10 yr after application, but did not affect soybean grain yields. The differences in response of the corn and soybean crops to gypsum might be related to the Ca²⁺ uptake by plants because of cation exchange properties of roots, being smaller for corn than for soybean. The use of gypsum in no‐till systems becomes more viable when corn is grown with a greater frequency in crop rotation.     

Long‐term development of nitrogen fluxes in a coniferous ecosystem: Does soil freezing trigger nitrate leaching?

In many forest ecosystems chronically large atmospheric deposition of N has caused considerable losses of inorganic N by seepage. Freezing and thawing of soil may alter the N turnover in soils and thereby the interannual variation of N seepage fluxes, which in turn makes it difficult to evaluate the N status of forest ecosystems. Here, we analyzed long‐term monitoring data of concentrations and fluxes of dissolved inorganic N (DIN) in throughfall and seepage from a Norway spruce stand at the Fichtelgebirge (SE Germany) between 1993 and 2004. Despite constant or even slightly increasing N inputs in throughfall, N losses with seepage at 90 cm declined from 15–32 kg N ha^–1 y^–1 in the first years of the study period (1993–1999) to 3–10 kg N ha^–1 y^–1 in 2000 to 2004. The large N losses in the first years coincided with extreme soil frost in the winter of 1995/96, ranging from –3.3°C to –1.0°C at 35 cm soil depth. Over the entire observation period, maximum fluxes of nitrate and ammonium were observed in the mineral soil following thawing of the soil. The elevated ammonium and nitrate fluxes resulted apparently from increased net ammonification and nitrification rates in the mineral soil, whereas mineral‐N fluxes in the O horizon were less affected by frost. Our data suggest that (1) extreme soil frost may cause substantial annual variations of nitrate losses with seepage and that (2) the assessment of the N status of forest ecosystems requires long periods of monitoring. Time series of biogeochemical data collected over the last 20–30 y include years with extreme cold winters and warm summers as well as unusual precipitation patterns. Analysis of such long‐term monitoring data should address climate extremes as a cause of variation in N outputs via leaching. The mean loss of 14.7 kg N with seepage water during 12 y of observation suggests that the forest ecosystem was saturated with N.     

Long‐term fertilization and manuring effects on physically separated soil organic‐matter pools under continuous wheat cropping at a rainfed semiarid site in China

An essential prerequisite for a sustainable soil use is to maintain a satisfactory soil organic‐matter (OM) level. This might be achieved by sound fertilization management, though impacts of fertilization on OM have been rarely investigated with the aid of physical fractionation techniques in semiarid regions. This study aimed at examining changes in organic C (OC) and N concentrations of physically separated soil OM pools after 26 y of fertilization at a site of the semiarid Loess Plateau in China. To separate sensitive OM pools, total macro‐OM (> 0.05 mm) was obtained from bulk soil by wet‐sieving and then separated into light macro‐OM (< 1.8 g cm^–3) and heavy macro‐OM (> 1.8 g cm^–3) subfractions; bulk soil was also differentiated into light OM (< 1.8 g cm^–3) and mineral‐associated OM (> 1.8 g cm^–3). Farmyard manure increased concentrations of total macro‐OC and N by 19% and 25%, and those of light fraction OC and N by 36% and 46%, compared to no manuring; both light OC and N concentrations but only total macro‐OC concentration responded positively to mineral fertilizations compared to no mineral fertilization. This demonstrated that the light‐fraction OM was more sensitive to organic or inorganic fertilization than the total macro‐OM. Mineral‐associated OC and N concentrations also increased by manuring or mineral fertilizations, indicating an increase of stable OM relative to no fertilization treatment, however, their shares on bulk soil OC and N decreased. Mineral fertilizations improved soil OM quality by decreasing C : N ratio in the light OM fraction whereas manuring led to a decline of the C : N ratio in the total macro‐OM fraction, with respect to nil treatment. Further fractionation of the total macro‐OM according to density clarified that across treatments about 3/4 of total macro‐OM was associated with minerals. Thus, by simultaneously applying particle‐size and density separation procedures, we clearly demonstrated that the macro‐OM differed from the light OM fraction not only in its chemical composition but also in associations with minerals. The proportion of the 0.5–0.25 mm water‐stable aggregates of soil was higher under organic or inorganic fertilizations than under no manure or no mineral fertilization, and increases in OC and N concentrations of water‐stable aggregates as affected by fertilization were greater for 1–0.5 and 0.5–0.25 mm classes than for the other classes. Results indicate that OM stocks in different soil pools can be increased and the loose aggregation of these strongly eroded loess soils can be improved by organic or inorganic fertilization.