Predicting long-term organic carbon dynamics in organically amended soils using the CQESTR model

Purpose  

The CQESTR model is a process-based C model recently developed to simulate soil organic matter (SOM) dynamics and uses readily available or easily measurable input parameters. The current version of CQESTR (v. 2.0) has been validated successfully with a number of datasets from agricultural sites in North America but still needs to be tested in other geographic areas and soil types under diverse organic management systems.     

Microbial biomass and mineralizable nitrogen distributions in no-tillage and plowed soils

Summary Distribution of soil microbial biomass and potentially mineralizable nitrogen (PMN) in long-term tillage comparisons at seven sites in the United States varied with tillage management and depth in soil. Microbial biomass and PMN levels of no-tillage soils averaged 54% and 37% higher, respectively, than those in the surface layer of plowed soil. Biomass and PMN levels were greatest in the surface 0 to 7.5-cm layer of no-tillage soil and decreased with depth in soil to 30 cm. Biomass and PMN levels of plowed soil, however, were generally greatest at the 7.5 –15 cm depth. Microbial biomass levels were closely associated with soil distributions of total C and N, water content, and water-soluble C as influenced by tillage management. Potentially mineralizable N levels in soil were primarily associated with distributions of microbial biomass and total N. Absolute levels of PMN and microbial biomass and the relative differences with tillage management were dependent on climatic, cropping, and soil conditions across locations. The additional N contained in soil biomass and PMN in the surface 0–7.5 cm of no-tillage compared with plowed soils ranged from 13 to 45 and 12 to 122 kg N/ha, respectively, for 6 of 7 locations. Fertilizer placement below the biologically rich surface soil layer and/or rotational tillage may improve short-term nitrogen use efficiency and crop growth on reduced-tillage soils.Contribution from USDA-ARS in cooperation with the Nebraska Agricultural Experimental Station, published as paper no. 8086, Journal Series, Nebraska Agricultural Experimental Station     

On-farm comparisons of soil organic carbon under no-tillage and chisel-plow systems

Purpose: Role of no-tillage (NT) in soil conservation has been already established but its influence on soil organic carbon (SOC) is still under debate.

Materials and methods: Three paired sites, with NT and chisel-plow (CT) fields adjacent to each other were selected for this study. Fields were under the same tillage practices for more than 20 years. Fields were sampled up to 90?cm depth to determine SOC and different C pools based on soil CO₂ flux during 86?d of incubation.

Results and conclusion: Significant differences in SOC and its pools were limited within the surface 0–15?cm depth only. Profile SOC did not vary between NT and CT. Tillage had a significant influence on soil C pools but the effect was not consistent across sites.     

Simulating trends in soil organic carbon of an Acrisol under no-tillage and disc-plow systems using the Century model

Soil organic matter (SOM) and its different pools have key importance in nutrient availability, soil structure, in the flux of trace gases between land surface and the atmosphere, and thus improving soil health. This is particularly critical for tropical soils. The rates of accumulation and decomposition of carbon in SOM are influenced by several factors that are best embodied by simulation models. However, little is known about the performance of SOM simulation model in an acid tropical soil under different tillage systems including no-tillage (NT). Our objective was to simulate soil organic matter dynamics on an Acrisol under no-tillage and different plowed systems using Century model. Tillage systems consisted of no-tillage, disc plow, heavy disc harrow followed by disc plow, and heavy disc harrow. Soil C stocks simulated by Century model showed tendency to recovery only under no-tillage. Also, simulated amounts of C stocks of slow and active pools were more sensitive to management impacts than total organic C. The values estimated by Century of soil C stocks and organic carbon in the slow and passive pools fitted satisfactorily with the measured data. Thus fitted, except for the active pool, Century showed acceptable performance in the prediction of SOM dynamics in an acid tropical soil.     

不同轮作制度下土壤中不稳定有机碳组分的变化

Taking Kenli County in the Yellow River Delta, China, as the study area and using digital satellite remote sensing techniques, cultivated land use changes and their corresponding driving forces were explored in this study. An interactive interpretation and a manual modification procedure were carried out to acquire cultivated land information. An overlay method based on classification results and a visual change detection method which was supported by land use maps were employed to detect the cultivated land changes. Based on the changes that were revealed and a spatial analysis between cultivated land use and related natural and socio-economic factors, the driving forces for cultivated land use changes in the study area were determined. The results showed a decrease in cultivated land in Kenli County of 5321.8 ha from 1987 to 1998, i.e., an average annual decrement of 483.8 ha, which occurred mainly in the central paddy field region and the northeast dry land region. Adverse human activities, soil salinization and water deficiencies were the driving forces that caused these cultivated land use changes.     

长期施肥条件下潮土耕层有机磷含量与组分的变化研究

以15年肥料长期定位试验为基础,采用Bowman-Cole有机磷分组方法,研究长期施肥对潮土耕层有机磷组分及其变化的影响.结果表明:耕层有机磷含量相对较低,有机磷总量为22.35～66.95 mg·kg-1,占土壤全磷的4.47%～9.86%.各组分中以中度活性有机磷为主要部分,占有机磷58.93%～82.40%.长期施肥对土壤各组分有机磷含量产生明显影响,与对照CK相比,PK处理活性有机磷和中度活性有机磷增加,NP处理活性有机磷和中度活性有机磷减少,两处理间差异显著.施用有机肥显著增加耕层土壤中度稳定性有机磷含量.耕层土壤碱性磷酸酶与中稳定性有机磷呈显著正相关.     

不同经营措施对毛竹林土壤有机碳含量及季节动态的影响

Soil samples for conventional management （CM） and intensive management （IM） practices were taken over a year at 2-month intervals to determine the effect of management practices on soil organic carbon （SOC） and to quantify seasonal dynamics in SOC for bamboo （Phyllostachys pubescens Mazel ex H. de Lehaie） stands. The results with IM compared to CM showed large decreases in total organic carbon （TOC）, microbial biomass carbon （MBC）, water-soluble organic carbon （WSOC）, and the MBC/TOC ratio in the soils. With all IM plots in the 0-20 cm depth across sampling periods, average decreases compared with CM were： TOC, 12.1%; MBC, 26.1%; WSOC, 29.3%; the MBC/TOC ratio, 16.1%; and the WSOC/TOC ratio, 20.0%. Due to seasonal changes of climate, seasonal variations were observed in MBC and WSOC. Soil MBC in the 0-20 cm depth in September compared to May were 122.9% greater for CM and 57.6% greater for IM. However, due primarily to soil temperature, soil MBC was higher during the July to November period, whereas because of soil moisture, WSOC was lower in July and January. This study revealed that intensive management in bamboo plantations depleted the soil C pool; therefore, soil quality with IM should be improved through application of organic manures.     

免耕与秸秆还田对中国农田土壤有机碳贮量变化的影响

农田碳汇管理措施的增汇效应是全球变化研究内容的重要命题之一.本文基于土地动态模拟系统(DLS)模拟了2012与2020年全国栅格尺度的农田分布,同时利用基于遥感反演的1988、2000年农田分布数据,应用CENTURY模型模拟了实施免耕、秸秆还田措施下中国1988～2020年农田土壤有机碳贮量的时空变化特征,分析了1988～2000年、2000～2012年与2012～2020年3个时段中国农田土壤有机碳贮量变化情况,揭示了九大农业生态区农田土壤有机碳贮量变化的时空分异特征.在不实施农田碳汇管理措施的情况下,1988～2000年全国大部分地区的农田土壤有机碳呈增长态势;而2000～2012年的全国农田土壤有机碳贮量出现一定幅度的下降.2012～2020年间,虽然全国范围内的土壤有机碳贮量表现为下降趋势,但下降幅度较2000～2012年间显著减少.研究结果表明,实施秸秆还田与免耕措施能够有效促进农田土壤有机碳贮量的增加,同时这两种管理措施的增汇效应具有显著的空间分异特征,黄淮海区、长江中下游区、华南区和西南区的增汇效果相对明显.上述研究结论为制订和实施农田增汇措施、减缓气候变化影响并保障农业发展的相关政策提供决策参考信息.     

Lability of soil organic carbon in tropical soils with different clay minerals

Soil organic carbon (SOC) storage and turnover is influenced by interactions between organic matter and the mineral soil fraction. However, the influence of clay content and type on SOC turnover rates remains unclear, particularly in tropical soils under natural vegetation. We examined the lability of SOC in tropical soils with contrasting clay mineralogy (kaolinite, smectite, allophane and Al-rich chlorite). Soil was sampled from A horizons at six sites in humid tropical areas of Ghana, Malaysian Borneo and the Solomon Islands and separated into fractions above and below 250 μm by wet sieving. Basal soil respiration rates were determined from bulk soils and soil fractions. Substrate induced respiration rates were determined from soil fractions. SOC lability was significantly influenced by clay mineralogy, but not by clay content when compared across contrasting clay minerals. The lability of SOC was lowest in the allophanic and chloritic soil, higher in the kaolinitic soils and highest in the smectitic soil. Our results contrast with conventional concepts of the greater capacity of smectite than of kaolinite to stabilize SOC. Contents of dithionite-citrate-bicarbonate extractable Fe and Al were inversely related to SOC lability when compared across soil types. A stronger inverse correlation between content of ammonium-oxalate extractable Fe and SOC lability was found when considering the kaolinitic soils only and we conclude that the content of active Fe (hydr-) oxides controls SOC stabilization in the kaolinitic soils. Our results suggest that the validity of predictive models of SOC turnover in tropical soils would be improved by the inclusion of soil types and contents of Fe and Al (hydr-) oxides.     

Soil organic carbon changes after 12 years of no-tillage and tillage of Grantsburg soils in southern Illinois

Many factors including management history, soil type, climate, and soil landscape processes affect the dynamics of soil organic carbon (SOC). The primary objective of this research was to determine the effects of no-tillage and tillage systems on the SOC content after 12 years of controlled treatments. A tillage experiment with three treatments (no-till (NT), chisel plow (CP) and moldboard plow (MP)) was initiated in the spring of 1989 in southern Illinois. The plot area was previously in a tall fescue hayland for 15 years and had a 6% slope. Maize (Zea mays L.) and soybean (Glycine max L. Merr.) were grown in the plot area on a yearly rotation system starting with maize. Periodically, the SOC content of various soil layers, to a depth of either 30 or 75 cm, was measured and expressed on both a gravimetric and volumetric basis. After 12 years, the 0–15 cm surface soil layer of MP was significantly lower in SOC than the NT and CP plots. For all but 2 values, the significance of findings did not change with the form of expression (gravimetric versus volumetric). The surface layer (0–15 cm), subsoil (15–75 cm), and rooting zone (0–75 cm) of all treatments had reduction in SOC on a volumetric basis when compared to the pre-treatment values for sod. At the end of the 12-year study, the MP system had significantly less SOC in the surface layer, subsurface layer and rooting zone than the NT system at comparable depths. After 12 years of tillage under a maize–soybean rotation, the NT treatment sequestered or maintained more SOC stock (47.0 Mt ha⁻¹) than the CP (43.7 Mt ha⁻¹) and MP (37.7 Mt ha⁻¹) treatments. The annual rate of SOC stock build up in the root zone (0–75 cm), above the MP system base, was 0.71 Mt ha⁻¹ year⁻¹ for the NT system and 0.46 Mt ha⁻¹ year⁻¹ for the CP system. For land coming out of the Conservation Reserve Program and returning to row crop production, NT and CP systems would maintain more SOC stock than MP system and reduce CO₂ emissions to the atmosphere.     

Spatial-temporal dynamics of organic carbon in soddy-podzolic soils of postagrogenic biogeocenoses

The organic carbon content in developed soddy-podzolic soils increased during the overgrowing of abandoned plowland with meadow and forest vegetation. The highest carbon content was recorded at the stage of 40–50-year-old forest, which was related to the largest input of organic matter into the soil and the intense litter decomposition during this period. A decrease in the soil carbon content was observed during the development of forest vegetation on the long-term hayfields in place of the former croplands, because the humus content in the lower part of the old-arable horizon decreased significantly. The spatial variability in the distribution of organic carbon in the soils increased with the development of forest biogeocenoses.     

Nitrogen transformations in tropical soils under conventional and sustainable farming systems

Samples of alluvial soil from mixed sandstone shale and slate and of Taiwan clay were collected from two sites, both managed under a similar crop rotation scheme. The fields were further divided into sections which were managed under either conventional farming or sustainable farming practices. When the soil samples were collected in April 1989, after 1 year of operation under conventional or sustainable practices, the nitrification activities of both soils managed under sustainable practices practices. The nitrifying activities in Taiwan clay samples collected in April 1993 which had been managed with chemical or with organic fertilizer were not significantly different. However, nitrifying activity in the alluvial soil was higher under sustainable than under conventional practices. Numbers of NH ₄ ⁺ -oxidizing bacteria were not significantly different in any of the soil samples irrespective of the different management practices. In contrast, higher numbers of NO ₂ ^- -oxidizing bacteria were detected in both soils managed sustainably. The results also indicated that the composition of NH ₄ ⁺ -oxidizing bacteria differed in the alluvial soil when managed with different kinds of fertilizer.     

Modeling soil organic carbon dynamics in Oxisols of Ibirubá (Brazil) with the Century Model

Introduction of conservation practices in degraded agricultural land will generally recuperate soil quality, especially by increasing soil organic matter. This aspect of soil organic C (SOC) dynamics under distinct cropping and management systems can be conveniently analyzed with ecosystem models such as the Century Model. In this study, Century was used to simulate SOC stocks in farm fields of the Ibirubá region of north central Rio Grande do Sul state in Southern Brazil. The region, where soils are predominantly Oxisols, was originally covered with subtropical woodlands and grasslands. SOC dynamics was simulated with a general scenario developed with historical data on soil management and cropping systems beginning with the onset of agriculture in 1900. From 1993 to 2050, two contrasting scenarios based on no-tillage soil management were established: the “status quo” scenario, with crops and agricultural inputs as currently practiced in the region and the “high biomass” scenario with increased frequency of corn in the cropping system, resulting in about 80% higher biomass addition to soils. Century simulations were in close agreement with SOC stocks measured in 2005 in the Oxisols with finer texture surface horizon originally under woodlands. However, simulations in the Oxisols with loamy surface horizon under woodlands and in the grassland soils were not as accurate. SOC stock decreased from 44% to 50% in fields originally under woodland and from 20% to 27% in fields under grasslands with the introduction of intensive annual grain crops with intensive tillage and harrowing operations. The adoption of conservation practices in the 1980s led to a stabilization of SOC stocks followed by a partial recovery of native stocks. Simulations to 2050 indicate that maintaining “status quo” would allow SOC stocks to recover from 81% to 86% of the native stocks under woodland and from 80% to 91% of the native stocks under grasslands. Adoption of a “high biomass” scenario would result in stocks from 75% to 95% of the original stocks under woodlands and from 89% to 102% in the grasslands by 2050. These simulations outcomes underline the importance of cropping system yielding higher biomass to further increase SOC content in these Oxisols. This application of the Century Model could reproduce general trends of SOC loss and recovery in the Oxisols of the Ibirubá region. Additional calibration and validation should be conducted before extensive usage of Century as a support tool for soil carbon sequestration projects in this and other regions can be recommended.     

Aggregate associated-C and physical protection in a tropical clayey soil under Malagasy conventional and no-tillage systems

Share-ploughed tillage with residue removed (CT-R) is the traditional tillage practice in the Highlands of Madagascar. No-tillage with residue mulching (NT+R) is nowadays often used as an alternative cultivation practice. Soils (0–5 cm layer) were sampled in Spring 2003 from both management systems after 11 years of soybean–maize annual rotation on a clayey Ferralsol. Soil aggregate stability can influence soil organic carbon (SOC) storage by its protection from microbial decomposition. The soil organic carbon (SOC) content was significantly impacted by systems and crop residues derived-carbon represented 64% of the annual benefit in SOC of NT+R system. The carbon associated with soil water stable macro- (200–2000 μm), meso- (20–200 μm) and microaggregates (<20 μm) from both systems, and their physical protection was studied by an incubation experiment of intact vs. crushed aggregates. Results showed macroaggregate content was significantly higher in NT+R than in CT-R system and mesoaggregate content was significantly higher in CT-R than in NT+R. Macroaggregates associated-C were 1.8 time higher in NT+R than in CT-R (31.9 and 17.9 g C g⁻¹ soil, respectively) and made up the largest percentage (>80%) of the difference of SOC content between NT+R and CT-R systems. The amount of mineralized C over 28 days was higher in NT+R than in CT-R, and higher in meso- than in macroaggregates. However, crushing aggregates did not significantly affect the amount of mineralized C in macro- and mesoaggregates for both management systems. The macro- and mesoaggregates protected-C was lower than 54 μg g⁻¹ soil for both NT+R and CT-R systems. Hence, the physical protection of C in aggregate larger than 50 μm was not the main process of C protection in the studied systems. Thus, C protection might occur in aggregates larger or smaller than 50 μm via physico-chemical protection mechanisms by association of organic matter to clay and silt fractions, or by protection due to chemical composition.     

Factor controlling soil organic carbon and total nitrogen dynamics under long-term conventional cultivation in seasonally frozen soils

Abstract

Limited information is available for understanding factors controlled dynamics of soil organic carbon (SOC) and total nitrogen (TN) affected by long-term conventional cultivation in seasonally frozen soils. A 19-year observation in this study was conducted in north-eastern China to evaluate effects and relative importance of potential factors. SOC variation extent was greater relative to global average as per unit of annual mean air temperature and precipitation changed. Increased carbon sequestration was observed in meadow lessive, while slight to moderate declines occurred in meadow-boggy soil and meadow soil. However, no differences in TN were found across soil types. At sites with low slope, carbon and nitrogen sequestration increased, largely due to water movement. Increased biomass with introducing 1-year oilseed rape/fallow in crop rotations could promote SOC and TN accumulation in the long run. Planting proportion of crops could also regulate carbon and nitrogen levels at a farm scale; the optimal ratio was observed in the range of 0.8–1.4. High crop yield was associated with lower carbon and nitrogen levels, and nutrient thresholds of yielding increment were observed as 25.7 g kg^?1 for carbon and 2.6 g kg^?1 for nitrogen. The length of frost-free period or cultivation period could not help sequestrating carbon and nitrogen. Chemical fertilizer with crop residues provoked SOC and TN increments compared with no chemical fertilizer plus little organic manure. Different factors exerted different tendentious influences, leading to subtle differences in SOC and TN variation rates. Accordingly, optimal cultivation strategies could be developed to reduce nutrient losses and mitigate greenhouse gas emissions.     

Simulation of soil organic carbon dynamics under different pasture managements using the RothC carbon model

Recently, soil carbon sequestration in agro-ecosystems has been attracting significant interest as soil organic carbon (SOC) can potentially offset some atmospheric carbon dioxide. The objectives of this study were to use the RothC model to simulate soil carbon sequestration and determine the proportion of pasture production as carbon input for SOC sequestration under different pasture types and pasture management in a long term experiment established in 1992. There were two types of pastures, annual and perennial pastures, with or without application of limestone. Simulation results showed that with an initial setting for the stubble retention factor of 0.65 and root/shoot ratio of 0.5 for annual pasture and 1.0 for perennial pasture, RothC can adequately simulate SOC for both pasture types, especially annual pasture. Using an inverse modelling technique, the root/shoot ratio was determined as 0.49 and 0.57 for annual pasture and 0.72 and 0.76 for perennial pasture with and without limestone application, respectively. There was a large improvement in model performance for perennial pasture with and without limestone application. The root mean squared errors (RMSE) reduced from 3.19 and 2.99 t C ha⁻¹ in the initial settings to 2.09 and 2.10 t C ha⁻¹, while performance efficiency (PE) increased from 0.89 and 0.91 to the same value of 0.95 when the root/shoot ratio of 0.72 and 0.76 were used for limed and unlimed perennial pastures. However, there was little improvement for annual pasture as RMSE had little change and PE was the same. As the stubble retention factor and root/shoot ratio can be combined into one factor that measures an equivalent amount of total above-ground pasture production allocated for soil carbon input, the modelled results can be summarised as 1.2 times and 1.4 times the above-ground dry matter for annual and for perennial pasture, respectively, regardless of liming. Our results provide useful information for simulation of soil carbon sequestration under continuous pasture systems.     

长期施肥下褐土易氧化有机碳及有机碳库的变化特征

本研究探讨了24年长期施肥对褐土土壤有机碳(TOC)、有机碳储量(TOCs)、净固碳效率(NCSE)和碳库管理指数(CPMI)的影响,为评价褐土土壤碳库变化与质量及科学施肥提供理论依据。研究以褐土肥力与肥料长期定位试验为平台,通过9个处理[A组:不施肥处理(N_0P_0、CK);B组:单施无机肥处理(N_1P_1、N_2P_2、N_3P_3和N_4P_4);C组:有机肥与无机肥配施处理(N_2P_1M_1、N_3P_2M_3和N_4P_2M_2);D组:单施高量有机肥处理(M_6)]测定土壤TOC与易氧化有机碳(ROOC)含量,并计算TOCs、NCSE及CPMI等相关指标。结果表明,在不同土层不同时期施用较高量有机肥配施无机肥及施用高量有机肥(N_3P_2M_3、N_4P_2M_2和M_6)均可提高TOC和ROOC含量,且随土层深度加深提升作用减弱。TOCs、NCSE与0~20 cm土层TOC含量在时间和空间上的变化规律基本一致。施用高量有机肥(C组、D组)可有效提高TOCs,A组、B组的TOCs均值分别比C组、D组低76.77%与17.36%。长期施肥处理可提高NCSE,尤其是施用有机肥处理可显著提高NCSE。NCSE为D组C组A组=B组;D组NCSE为1 152.27 kg·hm~(-2)·a~(-1),是C组的2.51倍,B组的16.20倍。与试验前相比,C组和D组的CPMI无显著变化,且C组与D组间差异不显著,但A组与B组比试验前降低16.38~40.02。与A组(CK)相比,B组中N1P1处理与C、D组处理显著影响CPMI,提高了23.30~45.67。在0~40 cm土层CPMI与ROOC含量呈显著正相关,CPMI可以很好地指示有机碳的变化。可见,施用高量有机肥或者较高量有机肥与无机肥配施可极显著提高褐土土壤TOCs、NCSE和CPMI,即施用高量有机肥或者较高量有机肥与无机肥配施(N_3P_2M_3和N_4P_2M_2)有利于褐土有机碳的固存,可减少无机肥的施用量,使土壤性质向良性方向发展,培肥土壤。     

Soil carbon and nitrogen dynamics using stable isotopes in 19- and 10-year-old tropical agroforestry systems

Conversion of forests to agricultural land in the American tropics, through traditional agricultural practices such as shifting cultivation, has not been able to maintain stocks of soil organic carbon (SOC), and increasing population pressure has led to shortened fallow periods, causing further losses of soil fertility. However, land management practices such as agroforestry can provide a sustainable alternative to single cropping because of its ability to maintain or increase the SOC pool. This study quantified SOC and nitrogen (N) pools, gross SOC turnover, residue stabilization efficiency (RSE_AC) in the alley crop, soil δ¹³C partitioning, C₃-C abundance and δ¹⁵N dynamics in 19- and 10-year Gliricidia sepium and Erythrina poeppigiana alley cropping system. Each system was studied at two fertilizer levels (tree prunings only [−N or −A], and tree prunings plus chicken manure [+N], or Arachis pintoi as a groundcover [+A]), and was compared to a sole crop system. The SOC and N pools were significantly higher (p < 0.05) in the 19-year-old alley crop compared to the sole crop, but not significantly different (p < 0.05) in the 10-year-old system. Soil C and N (%) showed a similar trend as that of the SOC and N pools in both 19- and 10-year-old systems. Gross SOC turnover, to a 20 cm depth, ranged from 12 to 21 years in the 19-year-old alley crop compared to 50 years in the sole crop, and from 20 to 32 years in the 10-year-old alley crop compared to 106 years in the sole crop. The RSE_AC ranged from 10% to 58% in the 19-year-old system, and from 3% to 43% in the 10-year-old system. The δ¹³C signature of the soil shifted significantly (p < 0.05) towards that of C₃ vegetation in the alley crop due to the greater input of organic residues from tree prunings compared to the sole crop. The proportion of input from tree prunings only in the 19-year-old alley crop ranged from 14% to 20%, and from 9% to 11% in the 10-year-old system to a soil depth of 20 cm. The δ¹⁵N signature of the soil showed two patterns: that of the 19-year-old system being enriched in δ¹⁵N, and that of the 10-year-old system being depleted in δ¹⁵N compared to the sole crop. The addition of manure in the 19-year-old system has enriched the soil δ¹⁵N and in the 10-year-old system the soil was depleted due to the N₂-fixing groundcover A. pintoi.     

Long-term changes in soil organic matter under conventional tillage and no-tillage systems in semiarid Morocco

Abstract. A no-tillage (NT) system was developed in semiarid Morocco to improve the soil fertility and stabilize yield through conservation of water. Results in two long-term trials (4 and 11 years) were able to show the effects of a no-tillage system in increasing total soil organic matter and total nitrogen. Over time, the quality of the NT soil surface was improved compared with that under conventional tillage (CT) with disc harrows. This effect was the result of an increase in soil organic carbon (SOC) and a slight decline in pH. However, over time, nitrogen decreased in both tillage practices, especially in the 0–25 mm layer (from 0.59 to 0.57 t ha⁻¹ and from 0.44 to 0.42 t ha⁻¹ under NT and CT, respectively). After 4 years of NT an extra 5.62 t ha⁻¹ of SOC was sequestered in the 0–25 mm layer, and after 11 years the SOC increased further to 7.21 t ha⁻¹.     

Modeling carbon dynamics in subsoils using simple models

Soil C dynamics below the plow layer have been little studied, despite a suspected large C‐stabilization potential of subsurface horizons. The objective of this study was to test two simple models (model A: single compartment for C₃‐ and for C₄‐derived C; model B: division of C₃‐ and C₄‐derived C into active and passive compartments) in their ability to simulate the C dynamics in subsoil horizons of a Haplic Phaeozem after conversion from C₃ (rye) to C₄ cropping (maize). The models were calibrated on an unfertilized maize soil and then validated on a maize soil with NPK fertilization. Both models simulated well C₃‐C and C₄‐C dynamics in the investigated soil depths (20–40 cm and 40–60 cm). In all cases, the model efficiency EF was > 0, which indicated that the simulated values described the trend in the measured data better than the mean of the observations. However, we observed some inconsistency in the obtained parameter set (e.g., a higher proportion of passive C for C₄‐derived than for C₃‐derived C or a very low decomposition rate constant for passive C₄‐C in 40–60 cm), which we assume to result from data restrictions on the investigated soils. More detailed data on SOC pools and turnover rates in subsoils which are generally not yet available for most experimental plots is vitally needed—especially for applying more sophisticated C‐dynamics models.