玉米生长对石灰性土壤无机碳与有机碳释放的根际效应

利用IsoSource模型三源区分玉米根际土壤CO_2释放来源(根源呼吸、土壤无机碳与有机碳释放),研究玉米根际效应对石灰性土壤无机碳与有机碳释放的影响。在玉米拔节期(24～53 d)、抽穗期(54～66 d)和灌浆期(67～99 d)末分别破坏性取样,测定根系、土壤有机碳和无机碳的~(13)C含量等指标;自拔节期开始至生育期末,每隔3d测定种植玉米与不种玉米的土壤呼吸CO_2量以及13C-CO_2含量。结果表明,利用IsoSource软件三源区分土壤CO_2的排放,土壤CO_2排放累计量以根源呼吸贡献为主(48.0%),其次为土壤有机碳(31.2%),最小为土壤无机碳(20.8%)。玉米对土壤无机碳与有机碳释放均表现为正根际效应,从拔节期至生育期末,种植玉米土壤有机碳与无机碳的释放分别较不种植土壤多65%和156%。土壤无机碳对于稳定全球碳库和调节大气CO_2浓度具有重要意义,若忽视石灰性土壤无机碳对土壤CO_2释放的贡献,有可能高估土壤有机碳的分解。     

玉米生长对石灰性土壤无机碳与有机碳释放的根际效应

利用IsoSource模型三源区分玉米根际土壤CO_(2)释放来源（根源呼吸、土壤无机碳与有机碳释放），研究玉米根际效应对石灰性土壤无机碳与有机碳释放的影响。在玉米拔节期（24～53 d）、抽穗期（54～66 d）和灌浆期（67～99 d）末分别破坏性取样，测定根系、土壤有机碳和无机碳的~(13)C含量等指标；自拔节期开始至生育期末，每隔3d测定种植玉米与不种玉米的土壤呼吸CO_(2)量以及~(13)C-CO_(2)含量。结果表明，利用IsoSource软件三源区分土壤CO_(2)的排放，土壤CO_(2)排放累计量以根源呼吸贡献为主（48.0%），其次为土壤有机碳（31.2%），最小为土壤无机碳（20.8%）。玉米对土壤无机碳与有机碳释放均表现为正根际效应，从拔节期至生育期末，种植玉米土壤有机碳与无机碳的释放分别较不种植土壤多65%和156%。土壤无机碳对于稳定全球碳库和调节大气CO_(2)浓度具有重要意义，若忽视石灰性土壤无机碳对土壤CO_(2)释放的贡献，有可能高估土壤有机碳的分解。     

秸秆添加对石灰性土壤有机与无机碳释放的影响

在富含碳酸盐的石灰性土壤上,土壤本身CO2释放不仅来自土壤有机碳（SOC）的分解,也源于无机碳（SIC）的溶解。在秸秆还田下,石灰性土壤CO2释放来源达到三个（秸秆碳、SOC和SIC）,由于区分技术的限制,当前区分CO2释放三源的研究,尚少见报道。以华北石灰性农田土壤为研究对象,采用13C标记玉米秸秆添加土壤进行室内培养32周,设置4个处理,分别为无添加对照（CK）、低量秸秆添加（S1,相当于田间秸秆还田量9.6 t?hm-2）、中量秸秆添加（S2,秸秆还田量28.8 t?hm-2）和高量秸秆添加（S3,秸秆还田量48.0 t?hm-2）,利用秸秆碳、SOC与SIC之间的δ13C差异,借助稳定同位素溯源模型IsoSource,区分土壤CO2的释放来源,明确秸秆添加对石灰性土壤有机与无机碳释放的影响。结果表明,随着培养时间的进行,土壤释放CO2中源于秸秆的贡献呈下降趋势;秸秆分解对土壤CO2释放的贡献随着秸秆添加量增加而增加,对于S1、S2和S3处理,土壤释放CO2中源于秸秆、SOC和SIC的贡献比值约分别为3:3:4、5:2:3和6:2:2;与CK相比,S1处理降低SOC分解的激发效应（程度为9%）,S2和S3处理反而增加了SOC分解的激发效应（程度分别为22%和57%）;秸秆和SOC矿化增加SIC溶解的释放,随秸秆添加量增加而增加,S1、S2和S3处理提高SIC源CO2的释放程度分别为368%、561%和652%。因此,秸秆添加不仅影响SOC源CO2的释放,也增加了SIC源CO2的释放,若忽略SIC溶解对土壤CO2释放的贡献,可能导致SOC矿化量的高估,进而影响SOC激发效应评估的准确度。     

分根区交替灌溉对玉米水分利用和土壤微生物量碳的影响

分根区交替灌溉由于创造了一个土壤水分分布不均匀的环境,从而影响土壤中微生物活性,作物水分和养分利用。为探明这种影响,该文通过盆栽试验,研究了在2种灌水水平(正常灌水W1,70%～80%田间持水率;轻度缺水W2,60%～70%田间持水率)和2种有机无机氮比例(100%无机氮,70%无机氮+30%有机氮)条件下,常规灌溉和不同生育期分根区交替灌溉(分别在苗期～灌浆初期、苗期～拔节期以及拔节期～抽雄期进行分根区交替灌溉(AI),即AI1、AI2和AI3)对玉米干物质积累、水分利用以及拔节期、抽雄期和灌浆初期土壤微生物量碳(MBC),可溶性碳(DOC)含量以及基础呼吸和诱导呼吸CO2释放量等的影响。结果表明,与常规灌溉相比,轻度缺水时,拔节期～抽雄期分根区交替灌溉总干物质质量增加23.2%～27.4%,水分利用效率提高23.3%～26.7%;相同施肥和灌水水平条件下,抽雄期时拔节期～抽雄期分根区交替灌溉土壤MBC增加,但是土壤诱导呼吸CO2释放量降低。与单施无机氮相比,有机、无机氮配施增加玉米干物质质量,在某些水分条件下(W1CI、W1AI1和W1AI2)还提高灌浆初期基础呼吸和诱导呼吸CO2释放量。因此,轻度缺水时拔节期～抽雄期进行分根区交替灌溉可以提高玉米总干物质质量、水分利用效率和微生物量碳。     

内蒙农牧交错带地区土地利用方式和施肥对土壤碳库的影响

内蒙武川是我国典型的内蒙农牧交错带地区,土地利用方式转变和施肥是影响该地区农业生产和土壤碳储量的重要人类活动。选取内蒙武川地区,针对不同土地利用方式(耕地、退耕还林/还草)和施肥措施(化肥、有机肥)的长期定位试验土壤,分析土壤有机碳(SOC)、土壤无机碳(SIC)和全氮(TN)含量和储量,结合13C和15N稳定同位素方法,研究土地利用方式和施肥措施对于该地区土壤碳氮转化的影响规律。研究表明,退耕还灌/还草后,SOC储量较耕地均有显著提高(提高幅度0.60~0.98 Mg hm-2 a-1),SIC储量也增加或保持相同水平(柠条地除外)。相比不施肥处理,施用有机肥能显著增加SOC(1.08~1.19 Mg hm-2 a-1),施化肥处理则会降低SIC(0.06~0.16 Mg hm-2 a-1),且主要影响次生碳酸盐。施肥SIC中原生碳酸盐比例(0~23%)低于自然土壤(3%~29%)。施肥措施对于土壤碳氮的转化强度远大于土地利用方式的改变。对于内蒙等干旱半干旱地区土壤,土地利用和施肥措施对于土壤有机和无机碳的影响应该在区域固碳管理中给予全面考虑。     

荒漠草原沙漠化对土壤无机碳和有机碳的影响

以空间代替时间的方法,通过对宁夏荒漠草原不同沙漠化阶段土壤有机碳(SOC)和无机碳(SIC)的研究,探讨荒漠草原沙漠化对土壤SIC、SOC及不同粒径组分土壤SIC、SOC分布特征的影响。结果表明:(1)随着荒漠草原沙漠化程度的加剧,0—10cm土层各粒径组分土壤SIC和SOC含量呈下降趋势。半固定沙地和流动沙地各粒径组分土壤SIC含量均表现为黏粉粒无机碳(CSIC)>细砂粒无机碳(FIC)>粗砂粒无机碳(CIC),而SOC含量均表现为细砂粒有机碳(FOC)>粗砂粒有机碳(COC)>黏粉粒有机碳(CSOC)。(2)随着荒漠草原沙漠化程度的加剧,0—30cm土层土壤无机碳(SICD)、土壤有机碳(SOCD)和土壤总碳(STCD)密度均表现为荒漠草原>固定沙地>半固定沙地>流动沙地。固定沙地、半固定沙地和流动沙地土壤SOCD、SICD分别比荒漠草原降低了18.5%,57.7%,60.5%和6.7%,35.9%,47.0%。(3)0—10cm土层各粒径组分土壤SOC和SIC含量、全土SOC含量与0—30cm土层SOC和SIC均呈显著正相关关系,其中土壤粗砂粒有机碳和粗砂粒无机碳对SOC影响最大,而土壤黏粉粒有机碳和黏粉粒无机碳与全土SIC含量呈显著负相关关系。因此,沙漠化防治对于减少荒漠草原土壤碳损失极为重要。     

黄土丘陵区不同恢复年限对天然草地土壤碳库动态的影响

[目的]揭示不同恢复年限的天然草地土壤碳库动态变化及其剖面分布特征,全面认识和理解天然草地恢复下土壤有机库、无机碳库的动态特征。[方法]采用野外调查与室内试验分析相结合的方法,以农田为对照,对黄土丘陵区不同恢复年限(11,16,22和35a)的天然草地土壤有机碳(SOC)、无机碳(SIC)、总碳(STC)的动态变化及其剖面分布特征进行了探讨。[结果](1)天然草地恢复过程中表层(0—10cm)SOC含量随植被恢复年限显著增加,下层(10—100cm)SOC含量随植被恢复年限变化不明显;0—100cm土层SOC储量呈先减少后增加趋势,但仍未达到农田SOC储量的水平。(2)天然草地0—20cm土层SIC含量呈相对脱钙现象,0—100cm土层SIC库储量约为SOC库储量的2.7~4.5倍。土壤无机碳库随植被恢复年限的增加无明显变化,但SIC的剖面分布深度发生改变。(3)土壤总碳库随恢复年限增加无明显变化,0—100cm土层SIC储量在STC库中所占比例约为75.6%~86.0%。[结论]短时间内天然草地的土壤碳汇效应并不明显,碳库增汇效应需要长期的过程。     

不同土地利用方式对土壤有机无机碳比例的影响

【目的】 土壤有机碳 (SOC) 和无机碳 (SIC) 对全球碳循环和减缓气候变化具有重要作用,进一步明确二者之间相互转化关系,对准确估算土壤碳储量具有重要意义。现有研究对SOC和SIC相互关系缺乏系统量化,研究结果不一。因此,明确SOC和SIC之间相互关系,可为准确估算和模拟土壤碳的转化过程提供理论基础。 【方法】 本研究搜集了我国1990—2018年已发表的文献共41篇,从不同气候区、不同土地利用方式、不同土层深度探究了SOC和SIC比例的变化,进一步量化了二者之间的相互关系。 【结果】 不同气候区、不同土地利用方式下土壤SOC/SIC值在0—20 cm土层均大于20—100 cm土层。具体来说,在温带大陆性气候区,草地0—20 cm土壤SOC/SIC值最小 (0.53),林地 (0.90) 和农田 (0.80) 土壤较高,且三种土地利用方式下SOC和SIC呈极显著正相关关系;而在温带季风性气候区,0—20 cm土壤SOC/SIC值表现为草地 (0.82) ≈ 农田 (1.05) > 林地 (0.29),且SOC和SIC在林地、农田土壤中呈正相关关系,但在草地土壤中二者为负相关关系。另外,温带大陆性气候区20—100 cm以林地土壤SOC/SIC值最高,草地和农田次之,而在温带季风性气候区三种土地利用方式下无显著差异;SOC和SIC在林地和农田土壤中呈正相关关系,然而在草地土壤中为负相关关系。温带大陆性气候区SOC/SIC值总体以林地较大,农田、草地次之。温带季风性气候区,0—20 cm土层SOC/SIC值以草地较大,农田和林地分别次之。这可能是因为植被覆盖不同,导致了作物碳的归还量不一。同时,不同的植被覆盖还影响了土壤中的各种生物化学进程,改变了碳在土壤中的循环转化过程,进而影响了SOC和SIC含量,使得SOC/SIC值产生较大差异。 【结论】 SOC和SIC之间存在循环转化关系,且不同气候条件、不同土地利用方式、不同土壤类型对SOC和SIC循环转化存在显著影响。不同条件下SOC/SIC值存在显著差异,且二者呈现不同的相关性。本研究结果可为明确土壤碳的循环积累机制,准确估算土壤有机和无机碳库提供理论依据。      

免耕对华北地区潮土碳库特征的影响

以实施7年的中国科学院禹城综合试验站冬小麦-夏玉米轮作免耕长期定位试验场为对象,系统研究免耕条件下土壤总碳(TC)、有机碳(SOC)、无机碳(SIC)的变化,为进一步评价免耕措施对华北地区潮土碳库的影响提供数据支持。研究设置免耕秸秆覆盖(NTRC)、免耕施用有机肥(NTRR)、常规耕作(CT)3种处理,分析表层(0-20cm)及深层(20-60cm)土壤TC、SOC及SIC的变化特征和影响因素。主要结果为:NTRC和NTRR能够增加0-20cm土层TC含量及储量,但降低20-60cm土层TC含量及储量,0-60cm总碳储量表现为NTRC>CT>NTRR;与CT相比,NTRC能够显著增加0-20cm而降低20-60cm土层SOC含量及储量,NTRR增加了0-5cm土层SOC含量及储量,在5-60cm则呈降低趋势,0-60cm土层SOC储量表现为CT>NTRC>NTRR;NTRC增加了0-60cm土层SIC储量,而NTRR则影响较小。TC与SOC呈显著正相关(P<0.05),而与SIC呈显著负相关(P<0.05),说明总碳的变化趋势与SOC一致,与SIC相反。     

不同施肥处理对光合碳在花生-土壤系统中分配的影响

通过室内花生盆栽,设置NPK(常规氮磷钾施肥)、NPKS(常规氮磷钾加玉米秸秆)、NPKA(常规氮磷钾加腐殖酸)和CK(不施肥对照)4个不同的施肥处理,采用3次~(13)CO2脉冲标记的方法对不同施肥处理下光合碳在花生-土壤系统中的分配进行定量研究。结果表明:不同施肥处理对标记期内花生总生物量影响不显著,但是NPKA处理显著提升了花生根系生物量,较CK、NPK和NPKS分别高22.04%、19.47%和53.38%。NPKS处理地上部~(13)C丰度最高,但土壤中~(13)C丰度最低,NPKA处理土壤中~(13)C丰度最高。各处理地上部的~(13)C含量无显著差异,NPKA处理根系的~(13)C含量显著高于NPK且土壤~(13)C含量显著高于其他处理。NPKA处理地上部的~(13)C分配比例最低而土壤中分配比例最高,根系~(13)C分配比例与其他处理无显著差异,根系与土壤~(13)C分配比例之和显著高于其他处理。本研究表明腐殖酸能显著促进花生光合碳向地下部的转运。     

CO2 emission and source partitioning from carbonate and non-carbonate soils during incubation

The accurate quantification and source partitioning of CO₂ emitted from carbonate (i.e., Haplustalf) and non-carbonate (i.e., Hapludult) soils are critically important for understanding terrestrial carbon (C) cycling. The two main methods to capture CO₂ released from soils are the alkali trap method and the direct gas sampling method. A 25-d laboratory incubation experiment was conducted to compare the efficacies of these two methods to analyze CO₂ emissions from the non-carbonate and carbonate-rich soils. An isotopic fraction was introduced into the calculations to determine the impacts on partitioning of the sources of CO₂ into soil organic carbon (SOC) and soil inorganic carbon (SIC) and into C₃ and/or C4 plant-derived SOC. The results indicated that CO₂ emissions from the non-carbonate soil measured using the alkali trap and gas sampling methods were not significantly different. For the carbonate-rich soil, the CO₂ emission measured using the alkali trap method was significantly higher than that measured using the gas sampling method from the 14th day of incubation onwards. Although SOC and SIC each accounted for about 50% of total soil C in the carbonate-rich soil, SOC decomposition contributed 57%–72% of the total CO₂ emitted. For both non-carbonate and carbonate-rich soils, the SOC derived from C4 plants decomposed faster than that originated from C₃ plants. We propose that for carbonate soil, CO₂ emission may be overestimated using the alkali trap method because of decreasing CO₂ pressure within the incubation jar, but underestimated using the direct gas sampling method. The gas sampling interval and ambient air may be important sources of error, and steps should be taken to mitigate errors related to these factors in soil incubation and CO₂ quantification studies.     

Effect of exogenous substances on soil organic and inorganic carbon sequestration under maize stover addition

ABSTRACT

Soil organic carbon (SOC) and inorganic carbon (SIC) are important carbon reservoirs in terrestrial ecosystems. A large portion of carbon from stover enters the atmosphere after stover return. However, there is little information on soil carbon sequestration during stover decomposition. In this study, a 54-day incubation experiment was conducted in calcareous soil to investigate the effects of wood ash or oil shale application (1.2 w/w%) on CO₂ emissions, soil C content, and other soil chemical properties. Four treatments were compared: (i) no maize stover addition; (ii) 1.5% maize stover; (iii) 1.5% maize stover plus 1.2% wood ash; and (iv) 1.5% maize stover plus 1.2% oil shale. Wood ash addition decreased CO₂ emission as a result of enhanced SIC sequestration in soil amended with maize stover; oil shale enhanced SOC due to increased carbon input from recalcitrant oil shale. Wood ash addition also significantly increased soil pH and soil microbial biomass carbon. The addition of wood ash to soil may be a potential strategy for promoting inorganic carbon storage and mitigating CO₂ emissions after stover return. In addition, oil shale is a very stable C source and oil shale amendment could be an ef?cient, long-term strategy to sequester organic C in soils.     

The effect of growing soybean (Glycine max. L.) on N2O emission from soil

A pot experiment was conducted to investigate the effect of growing soybean on N₂O emission from soil. When soybean was growing in pots, the cumulative N₂O emission during the growing season was 2.26 mg N pot⁻¹, which was 5.9 times greater than that from the identical but unplanted pots (CK). However, the difference in N₂O fluxes between the two treatments was not significant until the grain-filling stage. Of the total N₂O emission, 94% took place during the period from grain-filling to ripening. Premature harvesting of the aerial parts of the plants at various growth stages substantially stimulated N₂O emission from the soil. These results implied that the process of symbiotic N fixation per se does not stimulate N₂O production or emission, but rather senescence and decomposition of the roots and nodules in the late growth stage. Therefore, additional N₂O would be emitted from the soil after harvesting of soybean with roots, litter, and residues left in situ.     

外加碳酸盐增加石灰性土壤密闭培养过程中CO2的释放量

The closed-jar incubation method is widely used to estimate the mineralization of soil organic C. There are two C pools (i.e., organic and inorganic C) in calcareous soil. To evaluate the effect of additional carbonates on CO2 emission from calcareous soil during closed-jar incubation, three incubation experiments were conducted by adding different types (CaCO3 and MgCO3 ) and amounts of carbonate to the soil. The addition of carbonates significantly increased CO2 emission from the soil; the increase ranged from 12.0% in the CaCO3 amended soil to 460% in the MgCO3 amended soil during a 100-d incubation. Cumulative CO2 production at the end of the incubation was three times greater in the MgCO3 amended soil compared to the CaCO3 amended one. The CO2 emission increased with the amount of CaCO3 added to the soil. In contrast, CO2 emission decreased as the amount of MgCO3 added to the soil increased. Our results confirmed that the closed-jar incubation method could lead to an overestimate of organic C mineralization in calcareous soils. Because of its effect on soil pH and the dissolution of carbonates, HgCl2 should not be used to sterilize calcareous soil if the experiment includes the measurement of soil CO2 production.     

Soil carbon dioxide emission from intensively cultivated black soil in Northeast China: nitrogen fertilization effect

Purpose

The aim of this study was to understand the effect of nitrogen fertilization on soil respiration and native soil organic carbon (SOC) decomposition and to identify the key factor affecting soil respiration in a cultivated black soil.

Materials and methods

A field experiment was conducted at the Harbin State Key Agroecological Experimental Station, China. The study consisted of four treatments: unplanted and N-unfertilized soil (U0), unplanted soil treated with 225?kg?N?ha^?1 (UN), maize planted and N-unfertilized soil (P0), and planted soil fertilized with 225?kg?N?ha^?1 (PN). Soil CO₂ and N₂O fluxes were measured using the static closed chamber method.

Results and discussion

Cumulative CO₂ emissions during the maize growing season with the U0, UN, P0, and PN treatments were 1.29, 1.04, 2.30 and 2.27?Mg?C?ha^?1, respectively, indicating that N fertilization significantly reduced the decomposition of native SOC. However, no marked effect on soil respiration in planted soil was observed because the increase of rhizosphere respiration caused by N addition was counteracted by the reduction of native SOC decomposition. Soil CO₂ fluxes were significantly affected by soil temperature but not by soil moisture. The temperature sensitivity (Q ₁₀) of soil respiration was 2.16?C2.47 for unplanted soil but increased to 3.16?C3.44 in planted soil. N addition reduced the Q ₁₀ of native SOC decomposition possibly due to low labile organic C but increased the Q ₁₀ of soil respiration due to the stimulation of maize growth. The estimated annual CO₂ emission in N-fertilized soil was 1.28?Mg?C?ha^?1 and was replenished by the residual stubble, roots, and exudates. In contrast, the lost C (1.53?Mg?C?ha^?1) in N-unfertilized soil was not completely supplemented by maize residues, resulting in a reduction of SOC. Although N fertilization significantly increased N₂O emissions, the global warming potential of N₂O and CO₂ emissions in N-fertilized soil was significantly lower than in N-unfertilized soil.

Conclusions

The stimulatory or inhibitory effect of N fertilization on soil respiration and basal respiration may depend on labile organic C concentration in soil. The inhibitory effect of N fertilization on native SOC decomposition was mainly associated with low labile organic C in tested black soil. N application could reduce the global warming potential of CO₂ and N₂O emissions in black soil.     

The Origin of Soil Organic C,Dissolved Organic C and Respiration in a Long‐Term Maize Experiment in Halle,Germany, Determined by 13C Natural Abundance

For a quantitative analysis of SOC dynamics it is necessary to trace the origins of the soil organic compounds and the pathways of their transformations. We used the ¹³C isotope to determine the incorporation of maize residues into the soil organic carbon (SOC), to trace the origin of the dissolved organic carbon (DOC), and to quantify the fraction of the maize C in the soil respiration. The maize‐derived SOC was quantified in soil samples collected to a depth of 65 cm from two plots, one ’︁continuous maize’ and the other ’︁continuous rye’ (reference site) from the long‐term field experiment ’︁Ewiger Roggen’ in Halle. This field trial was established in 1878 and was partly changed to a continuous maize cropping system in 1961. Production rates and δ¹³C of DOC and CO₂ were determined for the Ap horizon in incubation experiments with undisturbed soil columns. After 37 years of continuous maize cropping, 15% of the total SOC in the topsoil originated from maize C. The fraction of the maize‐derived C below the ploughed horizon was only 5 to 3%. The total amount of maize C stored in the profile was 9080 kg ha⁻¹ which was equal to about 31% of the estimated total C input via maize residues (roots and stubble). Total leaching of DOC during the incubation period of 16 weeks was 1.1 g m⁻² and one third of the DOC derived from maize C. The specific DOC production rate from the maize‐derived SOC was 2.5 times higher than that from the older humus formed by C₃ plants. The total CO₂‐C emission for 16 weeks was 18 g m⁻². Fifty‐eight percent of the soil respiration originated from maize C. The specific CO₂ formation from maize‐derived SOC was 8 times higher than that from the older SOC formed by C₃ plants. The ratio of DOC production to CO₂‐C production was three times smaller for the young, maize‐derived SOC than for the older humus formed by C₃ plants.     

Carbon and nitrogen mineralization after maize harvest between and within maize rows: a microcosm study using 13C natural abundance

The sequestration of carbon in soil is not completely understood, and quantitative information about the rates of soil organic carbon (SOC) turnover could improve understanding. We analyzed the effects of the uneven distribution of crop residues after harvest of silage maize on C and N losses (CO₂‐C, dissolved organic carbon (DOC) and nitrogen (DON), and NO₃^–) from a Haplic Phaeozem and on the occurrence of priming effects induced by the decomposition of accumulated maize residues. Soil columns were taken from a continuous maize (since 1961) field after harvest i) between maize stalk rows (M_bare), ii) within the maize rows including a standing maize stalk (M_stalk), and iii) from a continuous rye (since 1878) field after tillage (rye stalk and roots were mixed into the Ap horizon). The soil columns were incubated for 230 days at 8 °C with an irrigation rate of 2 mm 10^–2 M CaCl₂ per day. Natural ¹³C abundance was used to distinguish between maize‐derived C (in SOC and maize residues) and older C originating from former C₃ vegetation. The uneven distribution of maize residues resulted in a considerably increased heterotrophic activity within the maize rows as compared with soil between seed rows. Cumulative CO₂ production was 53.1 g CO₂‐C m^–2 for M_stalk and 23.3 g CO₂‐C m^–2 for M_bare. The contribution of maize‐derived C to the total CO₂ emission was 83 % (M_stalk) and 67 % (M_bare). Calculated as difference between CO₂‐C release from M_stalk and M_bare, 19 % of the maize residues (roots and stalk) in M_stalk were mineralized during the incubation period. There was no or only a marginal effect of the accumulation of maize residues in M_stalk on leaching of DOC, DON, and NO₃^–. Total DOC and DON leaching amounted to 2.5 g C m^–2 and 0.16 g N m^–2 for M_stalk and to 2.1 g C m^–2 and 0.12 g N m^–2 for M_bare. The contribution of maize‐derived C to DOC leaching was about 25 % for M_stalk and M_bare. Nitrate leaching amounted to 3.9 g NO₃^–‐N m^–2 for M_stalk and to 3.5 g NO₃^–‐N m^–2 for M_bare. There was no priming effect induced by the decomposition of fresh maize residues with respect to CO₂ or DOC production from indigenous soil organic carbon derived from C₃ vegetation.     

Decomposition of C-labeled roots in a pasture soil exposed to 10 years of elevated CO2

The net flux of soil C is determined by the balance between soil C input and microbial decomposition, both of which might be altered under prolonged elevated atmospheric CO₂. In this study, we determined the effect of elevated CO₂ on decomposition of grass root material (Lolium perenne L.). ¹⁴C-labeled root material, produced under ambient (35 Pa pCO₂) or elevated CO₂ (70 Pa pCO₂) was incubated in soil for 64 days. The soils were taken from a pasture ecosystem which had been exposed to ambient (35 Pa pCO₂) or elevated CO₂ (60 Pa pCO₂) under FACE-conditions for 10 years and two fertilizer N rates: 140 and 560 kg N ha⁻¹ year⁻¹. In soil exposed to elevated CO₂, decomposition rates of root material grown at either ambient or elevated CO₂ were always lower than in the control soil exposed to ambient CO₂, demonstrating a change in microbial activity. In the soil that received the high rate of N fertilizer, decomposition of root material grown at elevated CO₂ decreased by approximately 17% after incubation for 64 days compared to root material grown at ambient CO₂. The amount of ¹⁴CO₂ respired per amount of ¹⁴C incorporated in the microbial biomass (q¹⁴CO₂) was significantly lower when roots were grown under high CO₂ compared to roots grown under low CO₂. We hypothesize that this decrease is the result of a shift in the microbial community, causing an increase in metabolic efficiency. Soils exposed to elevated CO₂ tended to respire more native SOC, both with and without the addition of the root material, probably resulting from a higher C supply to the soil during the 10 years of treatment with elevated CO₂. The results show the importance of using soils adapted to elevated CO₂ in studies of decomposition of roots grown under elevated CO₂. Our results further suggest that negative priming effects may obscure CO₂ data in incubation experiments with unlabeled substrates. From the results obtained, we conclude that a slower turnover of root material grown in an ‘elevated-CO₂ world’ may result in a limited net increase in C storage in ryegrass swards.     

Suppression of decomposition of 14C-labelled plant roots in the presence of living roots of maize and perennial ryegrass

The rates of decomposition of barley roots labelled with ¹⁴C were investigated in soil planted with maize or perennial ryegrass and in fallow controls. Evolution of ¹⁴CO₂ was significantly less from the planted soils than from fallow controls. Roots of maize and ryegrass appeared to compete substantially with soil microbes for ¹⁴C-labelled materials. Simple competitive effects were, however, insufficient to explain all of the observed effects of root growth on soil organic matter decomposition. There was no indication that the detrimental effects of maize roots on aggregate stability could be associated with increased degradation of native soil organic materials; the broader significance of the results is also discussed.     

Effect of land use types on decomposition of 14C-labelled maize residue (Zea mays L.)

The effect of three land use types on decomposition of ¹⁴C-labelled maize (Zea mays L.) residues and soil organic matter were investigated under laboratory conditions. Samples of three Dystric Cambisols under plow tillage (PT), reduced tillage (RT) and grassland (GL) collected from the upper 5 cm of the soil profile were incubated for 159 days at 20 °C with or without ¹⁴C-labelled maize residue. After 7 days cumulative CO₂ production was highest in GL and lowest in PT, reflecting differences in soil organic C (SOC) concentration among the three land use types and indicating that mineralized C is a sensitive indicator of the effects of land use regime on SOC. ¹⁴CO₂ efflux from maize residue decomposition was higher in GL than in PT, possibly due to higher SOC and microbial biomass C (MBC) in GL than in PT. ¹⁴CO₂ efflux dynamics from RT soil were different from those of PT and GL. RT had the lowest ¹⁴CO₂ efflux from days 2 to 14 and the highest from days 28 to 159. The lowest MBC in RT explained the delayed decomposition of residues at the beginning. A double exponential model gave a good fit to the mineralization of SOC and residue-¹⁴C (R² > 0.99) and allowed estimation of decomposition rates as dependent on land use. Land use affected the decomposition of labile fractions of SOC and of maize residue, but had no effect on the decomposition of recalcitrant fractions. We conclude that land use affected the decomposition dynamics within the first 1.5 months mainly because of differences in soil microbial biomass but had low effect on cumulative decomposition of maize residues within 5 months.