Results and discussion
Soil fractionation methodology
A key part of this dissertation was the soil fractionation, preceding the (isotope) analyses of total organic C and distinct biomarkers for plant (long-chain fatty acids) and microbial (amino sugars) origin. Density fractionation of bulk soil is performed to gain distinct soilfractions that relate to different stabilization mechanisms of soil OM (Golchin et al., 1994; Christensen, 2001; von Lützow et al., 2007; Crow et al., 2007; Wagai et al., 2009). However, a recent study showed that it is crucial to systematically determine the appropriate values of methodological parameters (density and dispersion energy) of the density fractionation procedure each time for different soils (Cerli et al., 2012). Nevertheless, looking into the literature, various different combinations of density and dispersion energy are used, but the criteria for their selection are obviously not transparent (Griepentrog & Schmidt, 2013). Therefore, a literature review was conducted and methodological parameters from recent studies that use density fractionation along with ultrasonic dispersion were compiled. Results showed that there are striking discrepancies in the application of density fractionation methodology (Griepentrog & Schmidt, 2013). Apparently, there is no consensus on which parameter values to use and the reasons for the selection of fractionation parameters are rarely explained. Thus, it is not known how results of different approaches relate to each other and it is almost impossible to compare results of different studies. It is therefore recommended to test and report effects of fractionation parameter values on chemical and physical properties of soilfractions, to achieve agreement and coherence on parameters to be used and facilitate comparability in future studies (Griepentrog & Schmidt, 2013).
In addition, the increased reactive nitrogen (N) deposition caused by the burning of fos- sil fuels and the use of artificial fertilizers (Davidson, 2009) may a ff ect large areas of the world in the future (Galloway, 2008). Excessive N deposition can negatively influence ecosystem health and species diversity (Aber et al., 1998), but lower concentrations can alleviate the N limitation that plants generally experience in grasslands, thereby
Because many grasslands are managed for feeding domes- tic herbivores, either directly through grazing or through for- age production, grassland C and N cycles might be affected because a large part of primary production is removed (Sous- sana et al., 2007). As a consequence, grasslands are of- ten fertilized with nutrients to sustain productivity. In addi- tion, the increased reactive nitrogen (N) deposition caused by the burning of fossil fuels and the use of artificial fertil- izers (Davidson, 2009) may affect large areas of the world in the future (Galloway, 2008). Excessive N deposition can negatively influence ecosystem health and species diversity (Aber et al., 1998), but lower concentrations can alleviate the N limitation that plants generally experience in grass- lands, thereby stimulating plant production (Lu et al., 2011). In their review, de Graaff et al. (2006) hypothesized that in- creased plant production in elevatedCO 2 could overcome in-
For the total yield, no significant difference was observed among CT, HT and RT but that of NT was lowest (Table 2). CT produced the highest mean winter wheat yield, but HT and RT produced higher summer maize yield from 2006 to 2012 compared to other treatments. The positive effect of CT on crop productivity was attributed to better physical and hydrological soil conditions (Mazzoncini et al. 2011). RT and HT did not perform well during the wheat seasons but increased maize yields, lead- ing to higher yields than CT in the total cropping season. There was an increase of wheat grain and total grain when N fertilization was increased from N0 to N3, and an increase of maize grain when N was increased from N0 to N2 (Table 2). The wheat yields treatments at N3 and N4 were 49.06% and 46.10% higher compared to N0 fer- tilization treatment, respectively. These results may be due to the straw returned to the soil that reduced N fertilizer losses and enhanced mineral N immobilization in the surface soil layers (Grageda- Cabrera et al. 2011). It indicates that the level of applied N can be reduced under suitable tillage or with straw returning (Habtegebrial et al. 2007).
Composite iron deposits containing non-conductivite Si particles and conductive Si-Fe alloy particles with an average size of 1μm were prepared in circulating solution, the effects of particle conductivity on the co-deposition process of particles were investigated. The results show that high conductivity contributed to the co-deposition process of particles into the composite coatings, and in a mode of "cladding" for the co-deposition of conductive Si-Fe particles, while that for non-conductive Si particles was "embedding" mode. At the same time, with increasing the conductivity, the co-deposition process of particles was obviously deviate from the Guglielmi model. A possible mechanism on the forming process was discussed.
Effects of different levels of compost applica- tion on the amounts and percentage distribution of organic N forms in whole soils and particle size fractions were investigated. Soil samples were collected from three plots: a) F, only chemical fertilizers; b) F + LC, chemical fertiliz- ers plus low level of compost; (c) F + HC, chemi- cal fertilizers plus high level of compost. Each soil sample was divided into five fractions: coarse sand-sized aggregate (CSA), medium sand-sized aggregate (MSA), fine sand-sized aggregate (FSA), silt-sized aggregate (SIA) and clay-sized aggregate (CLA) fractions. The sand fractions were subdivided into decayed plants (DP) and mineral particles (MP). The amounts of total N and different organic N forms in the whole soils as well as size fractions generally increased with increasing the amount of com- post. In the whole soils, percentage distribution of non-hydrolysable-N and amino sugar-N in- creased by compost application while the dis- tribution values of the hydrolysable ammonium- N and unidentified-N decreased. The application did not affect the distribution degree of amino acid-N. In the size fractions, the distribution values of most organic N forms increased in the CSA-DP, MSA-DP and FSA-DP fractions by compost application. In the CLA fractions, the a- mounts and percentage distribution of organic N forms were the highest, although the applica- tion caused decreases in their distribution val- ues. These findings indicate that the CLA frac- tion merit close attention as an important res- ervoir of various organic N.
matter are fundamental processes in ecosystem nutrient cycles, C flux, and humus formation  . Thus, invasive plants must have higher rates of litter decomposition obtain more nutrients for their further invasion, especially N. Invasive plants could also affect soil N cycles via the changed rates of litter decomposition. At the community level, litter decomposition rates are controlled by plants or functional type composition . Thus, plant invasion into a new ecosystem could pose a profound change in community structure and functions , mainly including changes in litter decomposition and nutrient cycles  . Plants could affect litter decom- position through direct and indirect ways. Direct effects are mediated via the quality of leaf litter, whereas indi- rect effects are related to unique conditions that the plants created in their surrounding environment . The concentration of litter N of invasive plants is generally often higher than that of native plants  . As one of the important indicators of litter quality, the concentration of litter N is one of the main factors affecting the rates of litter decomposition, i.e., high-quality litters that contain higher concentration of N tend to be decom- posed faster than low-quality litters that contain lower concentration of N  . Meanwhile, litter from ex- otics contains less lignin and lower lignin:N ratios than that of congeneric natives . Thus, the rates of litter decomposition of invasive plants are typically higher than that of native plants     . The dif- ferences in litter quality are often reflected by the amounts of microdecomposers colonizing it, and fungi typi- cally dominate microdecomposer communities on low-quality litters because of their lower nutrient require- ments and metabolic activities than those of bacteria  . However, increasing N could decrease the ratio of fungal and bacterial biomass - because
Nitrogen (N) is generally considered to be the most limiting element in temperate and boreal forests of North America (Kimmins and Feller, 1976; Krause, 1982; Vitousek and Matson, 1985; Binkley, 1986; Munson et al., 1993). The incorporation of nutrient-rich post-harvest forest residues can increase soil N and carbon (C) pools (Burger and Pritchett, 1984; Sanchez et al., 2000; Sanchez et al., 2003; Smethurst and Nambiar, 1990). Intensively managed pine plantation (Pinus taeda L.) sites in the southeastern U.S. are limited by low N or phosphorus (P) availability (Ducey and Allen 2001; Valentine and Allen 1990; Allen 1987), and low biomass productivity and therefore synthetic fertilizer applications have become common, and the fertilizer applications have increased forest floor accumulations (Gurlevick et al. 2003). Past research shows that typical shearing and piling of woody debris along with the movement of these materials off site may result in a site productivity decline due to a loss of site organic matter (Burger and Kluender, 1982; Burger and Pritchett, 1984). Hence, these on-site accumulated pools of post-harvest forest floor and slash have the potential to be large nutrient reservoirs for the subsequent pine plantation rotation.
Although the observed shifts in carbon accumulation were not large (cf. constrained and unconstrained ANCOVA mod- els), the allometric analysis showed that the potential for be- lowground shifts exists. The linear models used in the analysis of covariance fitted the data well (large R 2 values, small coef- ficients of variation) and the direction of the shifts agree with established concepts in the literature (Cannell 1989, Taiz and Zeiger 1991). Therefore, over several growing seasons the effects of altered carbon accumulation in response to elevated temperature and nitrogen availability could become important. The lack of a shift in belowground carbon accumulation in response to elevatedCO 2 suggests that, in loblolly and ponder- osa pine ecosystems at least, the standing pool of carbon in fine roots will not be altered by rising atmospheric CO 2 concentra- tions. However, a CO 2 response may be present in the form of altered flux of carbon to the soil from increased fine root production and turnover (Pregitzer et al. 1995).
Climate change is expected to alter the intensity and dynamics of soil freezing as a result of increased air temperatures and reduced snow cover. Soil freezing can influence ecosystem nitrogen (N) cycling by damaging plants and soil microorganisms, but little is known about how soil freezing effects on ecosystem N cycling may combine or interact with increased atmospheric N deposition, which is also expected to exert a strong influence on terrestrial ecosystems in the coming decades. The objective of my thesis was to examine the combined and possibly the interactive effects of climate induced changes in soil freezing and N addition on plant productivity, soil microorganisms, and soil nutrient cycling in a grass-dominated temperate old field ecosystem. First, using 15 N tracer, I investigated N retention by different nitrogen pools (plant, litter, roots, soil and simulated N deposition) in response to soil freezing under current and projected future atmospheric N deposition rates. My results indicated that soil freezing can increase N losses from soil over the winter and from atmospheric N deposition during the growing season, with the latter occurring due to decreased plant productivity. Second, I combined increased freezing (both in controlled environment chambers and in response to snow removal in the field) with N addition to explore whether soil freezing effects are mostly transient (i.e. over winter and spring melt), or whether there are legacy effects of freezing that continue over multiple years. My results indicated that the legacy effect of soil freezing reduced plant productivity over multiple years, but that N addition counteracted these declines in plant productivity. With respect to soil responses, freezing only caused short term (over winter) increases in extractable nitrogen pools, although there were also declines in fungal biomass during the second growing season as a legacy effect of freezing. Overall, my results indicate that intense soil freezing and increased atmospheric N deposition can both alter plant productivity and ecosystem N retention, although there were few significant interactions between these two factors.
Inappropriate applications of nitrogen (N) fertilizer have led to low N-use effi- ciencies (NUEs) and great N losses  , and have caused environmental problems, such as groundwater and surface water contamination, greenhouse gas increases, and soil quality degradation  . The movement and transfor- mation of N fertilizers in soils significantly affects the N supply, NUE and N loss. Therefore, research has focused on the migration and transformation of N in soils after fertilization   . Wang and Hou  reported that the migration of urea in soil was closely related to soil properties (especially the clay content) and the amount of urea applied. Additionally, the migration of N from urea oc- curred in a less than 10-cm zone in calcareous soil. Zhang et al.  suggested that the migration and transformation of urea and ammonium sulfate mainly occurs in the 0 - 5 cm soil layer in a black soil column. Recently, a soil column experiment also found that the transfer and nitrification of NH -N 4 + and
Soil organic matter can be analyzed on the basis of the different fractions. Changes in the levels of organic matter , caused by land use, can be better understood by alterations in the different fractions. Therefore in order to discover tendency of soil fertility sustainability it is significant to research on stable and labile form fractions of soil organic carbon by advanced methodology and modern technique. Our research work aimedto evaluate the effect of mineral and organic fertilizers on the labile and stable organic carbon of the chestnut soil in Mongolia. The soils samples used in this study we collected from variants of mineral ( N60P40K40), organic (biohumus 1t / hec.) Fertilizer and their combination of the Long-term fertilizers experiments of Plant and Agriculture Institute Changes in soil organic C by land use for agricultural purposes occurred mainly in the fraction of particulate organic matter (>20 µm ). The clay and silt fractions were quatified with a Mastersizer S after distruction organic substances and carbonates using H 2 O 2 and HCI and the sand fraction
in plant biomass with combined P and N addition (Fig. 1a, b) and a negative correlation between plant P uptake and moderate-cycling IP fractions (Fig. 6a, b). Therefore, the de- crease in soil moderate-cycling IP fractions with N addition could have been due to enhanced plant P uptake (Fig. 1c, d) as a result of increased plant biomass (Figs. 1a, b and 7). Under N addition, simultaneous increases in soil Olsen- P output (plant uptake) and input pathways (transformation from soil moderate-cycling IP fractions) may have resulted in mostly nonsignificant difference in soil Olsen-P concen- trations between combined P and N addition and P addi- tion alone (Fig. 5a, b). Nitrogen addition can potentially in- crease soil P availability by promoting solubilization of soil IP fractions in the short term (Wang et al., 2016). How- ever, long-term N deposition resulted in soil IP-exhaustion, thereby constraining the growth of plants (Olander and Vi- tousek, 2000; Yang et al., 2014). Previous research has also found that decades of N addition could accelerate PO 3− 4 re- lease (Malik et al., 2012; Stroia et al., 2011) and enhance conversion of soil recalcitrant IP fractions to soil labile IP and moderate-cycling IP fractions as a result of soil acidifi- cation (Alt et al., 2013). Additionally, N addition was found to suppress acid and alkaline phosphatase enzymes, result- ing in a decrease in soil organic P mineralization in a similar semiarid grassland (Tian et al., 2016). Therefore, combined N and P addition might decrease soil IP fractions by reduc- ing the conversion of soil organic P to IP as compared to P addition alone. Our results clearly illustrate that N effects on soil IP fractions depended on P inputs, where combined N and P additions could accelerate conversion of soil moderate- cycling IP fractions into soil-available P and enhance plant P uptake and biomass (Fig. 7).
proportion (0.1%) get introduced into non-native ranges and become invasive (Williamson and Fitter, 1996). Invasive plants can imperil native plant diversity (Enloe et al. 2004, Stinson et al. 2006, Stromberg et al. 2009), soil microbial communities (Lankau, 2011, Lekberg et al. 2013, Anthony et al. 2017), and entire structures of food webs (David et al. 2017). Land owners and managers in the U.S. spend approximately $120 billion dollars each year managing and covering costs incurred from invasive species (Pimental et al. 2005). While invasive species are a current global change stressor, they may become more of an issue in the future since the number of plants introduced each year is steadily rising (Seebens et al. 2018). Climate change could also exacerbate plant invasions by expanding the geographic range of current invasive species (Bradley et al. 2010) and making ecosystems intrinsically more vulnerable to new invasions (Dukes and Mooney, 1999). Some of the most aggressive invasive species in the U.S. already occupy >1,000,000 ha (Blaustein, 2001, Rodgers et al. 2008), and they will likely never be eradicated completely (Rejmánek and Pitcarin 2002). Thus, it is important to understand the efficacy of regional management and to design adaptive management for future conditions.
Embedded in the critical load concept is the idea that it is pos- sible to deﬁne a level of pollution that does not harm the natural environment. We test this concept for the deposition of reactive nitrogen (N). N deposition is recognized as one of the most serious pollution threats to global ecosystems (6), and it is ranked among the top ﬁve drivers of global biodiversity loss (7). Current critical loads for N deposition are primarily based on N addition experi- ments, which are valuable for identifying cause–effect relationships but have some limitations. Many experiments are located in regions with high ambient N deposition (often above the established critical load) (8) and use elevated N loads with few treatments over rela- tively short time periods. As a result, they are poorly suited to identifying the initial impacts of N deposition, vulnerable to treatment artifacts, and predisposed to short-term responses. Given these limits, there is a need to test experimentally derived critical load values with ﬁeld survey data. Field surveys also have limitations, but the strengths of each approach complement the drawbacks of the other. Properly constructed and analyzed, survey data along pollution gradients provide a means of testing the efﬁ- cacy of experimentally determined critical loads in the real world. If the experimentally derived critical load value adequately protects
Soilfractions derived from density fractionation have been argued to represent different pools of SOM with varying turnover times, accessibility by microbes and thus stability ( Crow et al., 2007; Schulze et al., 2009; von Lützow et al., 2007 ). Plant litter enters the SOM pool as free particulate organic matter (fPOM). fPOM is characterized by fast turnover times and low stability. It usually consists of easily degradable plant components such as cellulose, starch and other carbohydrates ( von Lützow et al., 2007 ), with smaller contributions by lignin, cutin and other compounds that are less decomposable. Products of enzymatic decomposition of fPOM as well as soluble litter components can be taken up by microbes or can become part of the mineral associated organic matter (MaOM), along with the now microbially transformed compounds. Usually the largest percentage of SOM is mineral- associated; this fraction is considered to have the longest time of up to >1000 years, depending on the minerals present ( Schulze et al., 2009 ). Mineral association also provides the greatest pro- tection from microbial decomposition ( Baisden et al., 2002; Poirier et al., 2005 ) and MaOM is sometimes even referred to as a passive pool (e.g. Gaudinski et al., 2000; Schulze et al., 2009 ). Both fPOM and MaOM can be incorporated in aggregates ( Poirier et al., 2005 ). While in aggregates POM is only slowly transformed and it is older and more stable than fPOM ( von Lützow et al., 2007 ). Based on their properties with respect to stability and persistence, the size, turn- over time, and chemical composition of these SOM pools, but also the transformations from one fraction to another might be affected by soil warming in different ways.
strategy is generally less effective than using cover crops (Justes et al. 1999) and also results in unpredictable duration of immobilisation (Thomsen and Christensen 1998). Use of cover crops and/or incorporation of arable residues to soil can also increase the risks of both intro- duction and seasonal carry-over of phytopathogens (Kumar and Goh 2000). Incorporation of exogenous materials to soil avoids the problems associated with pathogen persistence. Vinten et al. (1998) found that incorporation of waste paper showed good potential to immobilise soil-N, but repeated annual applications in- creased the concentrations of arsenic and nickel in the soil, and were thus unsuitable for agriculture. Rahn et al. (2009) showed that cardboard waste (combined with sugar-beet residues) could be a more favourable option, preventing about 40 % of N leaching compared to con- trols. Alternative approaches, such as the use of nitrifica- tion inhibitors can be effective in lowering nitrate leaching and N 2 O emissions (Menendez et al. 2012),
Data for soil N fractions were analyzed by using the Analysis of Covariance in the SAS-MIXED model, after considering STN in 2008 as a covariate (Littell et al. 2006 ). Weed management practice was consid- ered as the main plot treatment and a fixed effect, cropping sequence as the split plot treatment and another fixed effect, year as a repeated measure variable, and replication and replication 9 weed management practice interaction as random effects. Since W–P/B–F had three cropping phases with each phase of the cropping sequence occurring every year, data for soil N parameters were averaged across phases within the sequence, and the average value was used for the cropping sequence for analysis. Means were separated by using the least square means test when treatments and interactions were significant (Littell et al. 2006 ). Regression analysis was used to determine the rate of change of soil N fractions in treatments with year when their interactions were significant. The timings of 0.5, 1.5, and 2.5 year used in the regression analysis represent the actual periods of soil sampling in October 2009, 2010, and 2011, respectively, since the project was initiated in April 2009. Statistical significance was evaluated at P B 0.05, unless otherwise stated.
This work was supported by the Becas Chile Bicen- tenario Grant from CONICYT (Comisión Nacional de Investigación Científica y Tecnológica de Chile). We thank Mr. M. Kränzler, manager of the organic farm “Schönberger Hof”, and Dr. M. Mokry, Agricultural Technology Centre (LTZ) in Karlsruhe-Durlach, for pro- viding two of the investigated soils. The EUF analyses were performed by Dr. D. Horn, “EUF working group for the advancement of soil fertility and soil health” in Och- senfurt, Germany. The ICP-OES P analyses were carried out in the State Institute of Agricultural Chemistry of the University of Hohenheim.