Clay Addition to Sandy Soil Influence of Clay Type and Size on Nutrient Availability in Sandy Soils Amended with Residues Differing in C/N ratio

Addition of clay-rich subsoil to sandy soil results in heterogeneous soil with clay peds （2-mm） or finely ground （〈 2 mm） clay soil （FG）, which may affect the nutrient availability. The aim of this study was to assess the effect of clay soil particle size （FG or peds） and properties on nutrient availability and organic C binding in sandy soil after addition of residues with low （young kikuyu grass, KG） or high （faba bean, FB） C/N ratio. Two clay soils with high and low smectite percentage, clay and exchangeable Fe and A1 were added to a sandy soil at a rate of 20% （weight/weight） either as FG or peds. Over 45 d, available N and P as well as microbial biomass N and P concentrations and cumulative respiration were greater in soils with residues of KG than FB. For soils with KG residues, clay addition increased available N and initial microbial biomass C and N concentrations, but decreased cumulative respiration and P availability compared to sandy soil without clay. Differences in measured parameters between clay type and size were inconsistent and varied with time except the increase in total organic C in the 〈 53 μm fraction during the experiment, which was greater for soils with FG than with peds. We concluded that the high exchangeable Fe and A1 concentrations in the low-smectite clay soil can compensate a lower clay concentration and proportion of smectite with respect to binding of organic matter and nutrients.

Influence of salinity and water content on soil microorganisms

Salinization is one of the most serious land degradation problems facing world.Salinity results in poor plant growth and low soil microbial activity due to osmotic stress and toxic ions.Soil microorganisms play a pivotal role in soils through mineralization of organic matter into plant available nutrients.Therefore it is important to maintain high microbial activity in soils.Salinity tolerant soil microbes counteract osmotic stress by synthesizing osmolytes which allows them to maintain their cell turgor and metabolism.Osmotic potential is a function of the salt concentration in the soil solution and therefore affected by both salinity(measured as electrical conductivity at a certain water content)and soil water content.Soil salinity and water content vary in time and space.Understanding the effect of changes in salinity and water content on soil microorganisms is important for crop production,sustainable land use and rehabilitation of saline soils.In this review,the effects of soil salinity and water content on microbes are discussed to guide future research into management of saline soils.

Sorption of Water-Extractable Organic Carbon in Various Clay Subsoils: Effects of Soil Properties

Clay-rich subsoils are added to sandy soils to improve crop yield and increase organic carbon （C） sequestration; however, little is known about the influence of clay subsoil properties on organic C sorption and desorption. Batch sorption experiments were conducted with nine clay subsoils with a range of properties. The clay subsoils were shaken for 16 h at 4 ℃with water-extractable organic C （WEOC, 1 224 g C L-1） from mature wheat residue at a soil to extract ratio of 1：10. After removal of the supernatant, the residual pellet was shaken with deionised water to determine organic C desorption. The WEOC sorption was positively correlated with smectite and illite contents, cation exchange capacity （CEC） and total organic C, but negatively correlated with kaolinite content. Desorption of WEOC expressed as a percentage of WEOC sorbed was negatively correlated with smectite and illite contents, CEC, total and exchangeable calcium （Ca） concentrations and clay content, but positively correlated with kaolinite content. The relative importance of these properties varied among soil types. The soils with a high WEOC sorption capacity had medium CEC and their dominant clay minerals were smectite and illite. In contrast, kaolinite was the dominant clay mineral in the soils with a low WEOC sorption capacity and low-to-medium CEC. However, most soils had properties which could increase WEOC sorption as well as those that could decrease WEOC sorption. The relative importance of properties increasing or decreasing WEOC sorption varied with soils. The soils with high desorption had a low total Ca concentration, low-to-medium CEC and low clay content, whereas the soils with low desorption were characterised by medium-to-high CEC and smectite and illite were the dominant clay minerals. We conclude that WEOC sorption and desorption depend not on a single property but rather a combination of several properties of the subsoils in this study.

Soil Respiration, Microbial Biomass and Nutrient Availability in Soil After Addition of Residues with Adjusted N and P Concentrations

Microbial activity and nutrient release are known to be influenced by organic matter properties,but it is difficult to separate the effect of C/N ratio from that of C/P ratio because in most plant residues both ratios are either high or low.An incubation experiment was conducted to investigate the effects of reducing the C/N and C/P ratios of slowly decomposable plant residues(young eucalyptus leaves,mature wheat straw,and sawdust) to those of rapidly decomposable residues(young kikuyu shoots) on soil respiration,microbial biomass,and N and P availability.The C/N and C/P ratios of the former were adjusted to 15 and 89,respectively,by adding N as(NH_4)_2SO_4,P as KH_2PO_4 or both and residues were added at 10 g C kg-1 to a silt loam.Soil respiration was measured over21 d;microbial biomass C(MBC) and available N and P were measured on days 0,7,and 21.Compared to the unamended soil,addition of kikuyu increased cumulative respiration 20-fold,MBC concentration 4 to 8-fold,and available P concentration up to4-fold,whereas the increase in available N concentration was small and transient.Cumulative respiration and MBC concentration were low in the sawdust-amended soil and were not influenced by reducing the C/N and C/P ratios.Cumulative respiration with original wheat and eucalyptus was 30%-40%of that with kikuyu.Reducing the C/N ratio alone or both C/N and C/P ratios increased cumulative respiration and MBC concentration 2-fold compared to the original wheat and eucalyptus,whereas reducing the C/P ratio had little effect.Throughout the experiment,the available N concentration after addition of residues with reduced C/N ratio increased in the following order of eucalyptus < wheat < sawdust.By independently lowering the C/N and C/P ratios,microbial activity was more limited by C and N than P.However,lowering the C/N ratio of very slowly decomposable sawdust had no effect on soil respiration and MBC concentration,suggesting that other properties such as concentration of poorly decomposable C compounds limited decomposition.

Responses of Soil Microbial Activity and Biomass to Salinity After Repeated Additions of Plant Residues

Microbial adaptation to salinity can be achieved through synthesis of organic osmolytes,which requires high amounts of energy;however,a single addition of plant residues can only temporarily improve energy supply to soil microbes.Therefore,a laboratory incubation experiment was conducted to evaluate the responses of soil microbes to increasing salinity with repeated additions of plant residues using a loamy sand soil with an electrical conductivity in saturated paste extract(EC_e) of 0.6 dS m^(-1).The soil was kept non-saline or salinized by adding different amounts of NaCl to achieve EC_e of 12.5,25.0 and 50.0 dS m^(-1).The non-saline soil and the saline soils were amended with finely ground pea residues at two rates equivalent to 3.9 and 7.8 g C kg^(-1) soil on days 0,15 and29.The soils receiving no residues were included as a control.Cumulative respiration per g C added over 2 weeks after each residue addition was always greater at 3.9 than 7.8 g C kg^(-1) soil and higher in the non-saline soil than in the saline soils.In the saline soils,the cumulative respiration per g C added was higher after the second and third additions than after the first addition except with3.9 g C kg^(-1) at EC_e of 50 dS m^(_1).Though with the same amount of C added(7.8 g C kg^(-1)),salinity reduced soil respiration to a lesser extent when 3.9 g C kg^(-1) was added twice compared to a single addition of 7.8 g C kg^(-1).After the third residue addition,the microbial biomass C concentration was significantly lower in the soils with EC_e of 25 and 50 dS m^(_1) than in the non-saline soil at3.9 g C kg^(-1),but only in the soil with EC_e of 50 dS m^(-1) at 7.8 g C kg^(-1).We concluded that repeated residue additions increased the adaptation of soil microbial community to salinity,which was likely due to high C availability providing microbes with the energy needed for synthesis of organic osmolytes.

Phosphorus and nitrogen in the soil interface between two plant residues differing in C/nutrient ratio:A short-term laboratory incubation study

In studies on the effects of mixing residues with different properties on decomposition rate and nutrient release,the extent of contact between the different residues is not known.In this study,we used an experimental design where crop residues were spatially separated by a layer of soil.Microcosms were set up using young faba bean residue(low carbon(C)/nutrient ratio,L)and mature barley straw(high C/nutrient ratio,H).The microcosms comprised of two caps of PVC tubes,each filled with moist soil.Between the two caps,there were three layers each separated from the others by fine nylon mesh with the middle layer being the moist interface soil.Microcosms had similar(H/H or L/L)or different(L/H)residue types,or only residue type(H/S or L/S)while the other cap had no residue.In treatments with only one residue,measured parameters,except microbial biomass P(MBP),were higher in L/S than H/S.In treatments with two residues,all parameters were lowest in H/H.In L/H compared to L/L after 14 days,available P and microbial biomass N(MBN)were lower,available N was similar and MBP was higher.After 28 days,available P and N were lower in L/H than L/L,but MBP and MBN did not differ.In L/H,measured resin P,MBP and MBN were higher than expected whereas available N was lower.The experimental design used in this study allows assessing the effect of residues on properties of the soil between them.

Soil Respiration, Microbial Biomass C and N Availability in a Sandy Soil Amended with Clay and Residue Mixtures

Crop yields in sandy soils can be increased by addition of clay-rich soil, but little is known about the effect of clay addition on nutrient availability after addition of plant residues with different C/N ratios. A loamy sandy soil （7% clay） was amended with a clay-rich subsoil （73% clay） at low to high rates to achieve soil mixtures of 12%, 22%, and 30% clay, as compared to a control （sandy soil alone） with no clay addition. The sandy-clay soil mixtures were amended with finely ground plant residues at 10 g kg-l： mature wheat （Triticum aestivum L.） straw with a C/N ratio of 68, mature faba bean （Vicia faba L.） straw with a C/N ratio of 39, or their mixtures with different proportions （0% 100%, weight percentage） of each straw. Soil respiration was measured over days 0-45 and microbial biomass C （MBC）, available N, and pH on days 0, 15, 30, and 45. Cumulative respiration was not clearly related to the C/N ratio of the residues or their mixtures, but C use efficiency （cumulative respiration per unit of MBC on day 15） was greater with faba bean than with wheat and the differences among the residue mixtures were smaller at the highest clay addition rate. The MBC concentration was lowest in sole wheat and higher in residue mixtures with 50% of wheat and faba bean in the mixture or more faba bean. Soil N availability and soil pH were lower for the soil mixtures of 22% and 30% clay compared to the sandy soil alone. It could be concluded that soil cumulative respiration and MBC concentration were mainly influenced by residue addition, whereas available N and pH were influenced by clay addition to the sandy soil studied.

Impact of Salinity on Respiration and Organic Matter Dynamics in Soils is More Closely Related to Osmotic Potential than to Electrical Conductivity

Osmotic potential （OP） of soil solution may be a more appropriate parameter than electrical conductivity （EC） to evaluate the effect of salts on plant growth and soil biomass. However, this has not been examined in detail with respect to microbial activity and dissolved organic matter in soils with different texture. This study evaluated the effect of salinity and sodicity on respiration and dissolved organic matter dynamics in salt-affected soils with different texture. Four non-saline and non-sodic soils differing in texture （S-4, S-13, S-24 and S-40 with 4%, 13%, 24~ and 40~~ clay, respectively） were leached using combinations of 1 mol L-1 NaC1 and 1 mol L-1 CaC12 stock solutions, resulting in EC （1：5 soil：water ratio） between 0.4 and 5.0 dS m-1 with two levels of sodicity （sodium absorption ratio （SAR） 〈 3 （non-sodic） and 20 （sodic）, 1：5 soil：water ratio）. Adjusting the water content to levels optimal for microbial activity~ which differed among the soils, resulted in four ranges of OP in all the soils： from -0.06 to -0.24 （controls, without salt added）, -0.55 to -0.92, -1.25 to -1.62 and -2.77 to -3.00 Mpa. Finely ground mature wheat straw （20 g kg-1） was added to stimulate microbial activity. At a given EC, cumulative soil respiration was lower in the lighter-textured soils （S-4 and S-13） than in the heavier-textured soils （S-24 and S-40）. Cumulative soil respiration decreased with decreasing OP to a similar extent in all the soils, with a greater decrease on Day 40 than on Day 10. Cumulative soil respiration was greater at SAR ---- 20 than at SAR 〈 3 only at the OP levels between -0.62 and -1.62 MPa on Day 40. In all the soils and at both sampling times, concentrations of dissolved organic C and N were higher at the lowest OP levels （from -2.74 to -3.0 MPa） than in the controls （from -0.06 to -0.24 MPa）. Thus, OP is a better parameter than EC to evaluate the effect of salinity on dissolved organic matter and microbial activity in different textured soils.

Plant residues differing in C/N ratio in mulch and soil-the effect of the mulch on nutrient availability and microbial biomass is more pronounced with higher leaching amount

This study investigated the effect of C/N ratio,placement of plant residues,and leaching amounts on soil respiration,microbial biomass,and nutrient availability within four weeks after amendment.Young faba bean shoots(FB,C/N 7)and mature wheat straw(WH,C/N 80)were used as low and high C/N residue,respectively.Soil was unamended,mulched with FB or WH only,or mulched with one residue and mixed with the other residue.Leaching with 5 or 25 mL water was carried out on days 4,12,and 20.Cumulative respiration and microbial biomass N were higher with 25 than 5 mL only in treatments with two residues.WH under FB mulch reduced N availability compared to FB mulch alone,whereas FB under WH increased N availability compared toWHmulch alone.When soils were leached with 25 mL water,available N in FB mulch over WH was lower on day 12,but higher later,compared to WH mulch over FB.In contrast,in WH mulch over FB microbial biomass N increased over time whereas available N decreased.In conclusion,the effect of C/N ratio of the mulch on soil available and microbial biomass N was greater with the higher leaching amount.