View the figure 2 which shows the relationship between water filled pore space and relative soil respiration. What generalization can you make regarding the optimum soil water conditions for biological activity?Use the photos to answer the question figure 2 is in the photos USDA Natural Resources Conservation ServiceBFFunctionD/NSoil Quality IndicatorsSoil RespirationCarbon dioxide (CO2) release from the soil surface isreferred to as soil respiration. This CO2 results fromseveral sources, including aerobic microbial decompositionof soil organic matter (SOM) to obtain energy for theirgrowth and functioning (microbial respiration), plant rootand faunal respiration, and eventually from the dissolutionof carbonates in soil solution. Soil respiration is onemeasure of biological activity and decomposition. The rateof CO2 release is expressed as CO2-C lbs/acre/day (orkg/ha/d). It can be measured by simple field methods (e.g.fig. 1) or more sophisticated field and laboratory methods.During the decomposition of SOM, organic nutrientscontained in organic matter (e.g., organic phosphorus,nitrogen, and sulfur) are converted to inorganic forms thatare available for plant uptake. This conversion is known asmineralization. Soil respiration is also known as carbonmineralization.Factors AffectingInherent - Like all organisms, soil microbes haveoptimal conditions in which they thrive. Soil respirationrate depends on the amount and quality of SOM,temperature, moisture, and aeration. Biological activity ofsoil organisms varies seasonally, as well as daily. Microbialrespiration more than doubles for every 10°C rise up to anoptimum of about 35 to 40°C (95 to 104°F), beyond whichhigh temperature becomes limiting. On the other hand,SOM decomposition and microbial activity virtually ceasebelow about 5°C (41°F) (biological zero).Soil respiration increases with increasing soil moisture upto the level where low oxygen concentration (lack ofaeration) interferes with an organism's ability to res pire(fig. 2) The optimum soil moisture content for soilrespiration varies from site to site, but values as high as60% water-filled pore space have been reported. In drysoils, respiration declines because the soil moisture deficitlimits microbial activity. Conversely, in extremely wetsoils, low oxygen levels results in poor organic matterdecomposition and respiration rates. In these soils,anaerobic by-products are produced, such as methane orsulfides. Medium textured soils (silt soils) are oftenFigure 1. Draeger-TubesⓇ are used to measure CO2 released fromthe soil surface as an indicator of decomposition and soil respiration.favorable to soil respiration because of their good aerationand moisture content. In clay soils, a sizeable amount ofSOM is protected from decomposition by clay particlesand other aggregates. In reality, soil respiration responds tothe coupled action of temperature and moisture, especiallyto the most limiting of either factor.Dynamic Soil management practices that affectSOM, moisture, aggregation, and pH influence soilrespiration. Practices that leave crop residues at the soilsurface, such as no-till, use of cover crops, or otherpractices that add organic matter, usually promote soilrespiration. Crop residues with a low carbon to nitrogen(C:N) ratio, such as that from legumes, decompose fasterand produce higher CO2 rates than residues with a highC:N ratio (e.g. wheat straw). High C:N ratio crops coupledwith added N (from any source) increase decompositionand accrual of SOM. Practices that increase SOM alsoimprove soil aggregation and porosity, and therefore,aeration and soil moisture content. Conversely, continuouscultivation and other conventional tillage methods thatremove, bury, or burn crop residues diminish SOM contentand microbial activity by reducing aggregate stability andporosity, and increasing erosion of surface layers that arenormally highest in SOM and populations of organismsthat are the key to soil respiration. Irrigation in dryconditions and drainage of wet soils can significantly boostsoil respiration.Soil pH regulates nutrient availability and distribution,activity of soil organisms responsible for SOM20Helping People Help the Land... College LITION Adecomposition, and other processes contributing to soilrespiration. Chemical fertilizer may stimulate root growthand nourish microbes; however, at high concentrations,some fertilizers can become harmful to microbesresponsible for soil respiration because of changes in pHand their potential toxicity. Similarly, organic amendmentswith high concentrations of heavy metals, as well aspesticides and fungicides, may be toxic to microbialpopulations leading to reduced microbial diversity,abundance, and respiration.Relationship to Soil FunctionSoil respiration reflects the capacity of soil to support soillife including crops, soil animals, and microorganisms. Itdescribes the level of microbial activity, SOM content andits decomposition. In the laboratory, soil respiration can beused to estimate soil microbial biomass and make someinference about nutrient cycling in the soil. Soil respirationalso provides an indication of the soil's ability to sustainplant growth. Excessive respiration and SOMdecomposition usually occurs after tillage due todestruction of soil aggregates that previously protectedSOM and increased soil aeration. Depleted SOM, reducedsoil aggregation, and limited nutrient availability for plantsand microorganisms can result in reduced crop productionin the absence of additional inputs. The threshold betweenaccumulation and loss of organic matter is difficult topredict without knowledge of the amount of carbon added.Problems with Poor FunctionReduced soil respiration rates indicate that there is little orno SOM or aerobic microbial activity in the soil. It mayalso signify that soil properties that contribute to soilrespiration (soil temperature, moisture, aeration, availableN) are limiting biological activity and SOMdecomposition. With reduced soil respiration, nutrients arenot released from SOM to feed plants and soil organisms.This affects plant root respiration, which can result in thedeath of the plants. Incomplete mineralization of SOMoften occurs in saturated or flooded soils, resulting in theformation of compounds that are harmful to plant roots,(e.g. methane and alcohol). In such anaerobicenvironments, denitrification and sulfur volatilizationusually occur, contributing to greenhouse gas emissionsand acid deposition.Improving Soil RespirationThe rate of soil respiration under favorable temperatureand moisture conditions is generally limited by the supplyof SOM. Agricultural practices that increase SOM usuallyenhance soil respiration. The following practices have thepotential to significantly improve SOM and indirectly soilrespiration when other factors are at an optimum:Conservation tillage (no-till, strip-till, mulch till, etc.)Application of manure and other organic by-productsRotations with high residue and deep-rooted cropsCover and green manure cropsIrrigation or drainageControlled trafficMeasuring Soil RespirationSoil respiration is measured using the Draeger-TubeⓇmethod described in the Soil Quality Test Kit Guide,Chapter 2, p 4 - 6. See Section II, Chapter 1, p 52 - 54 forinterpretation of results.References:Parkin TB, Doran JW, and Franco-Vizcaíno E. 1996. Fieldand Laboratory Tests of Soil Respiration. In: Doran JW,Jones AJ, editors. Methods for assessing soil quality.Madison, WI. p 231-45.senstonZibilske LM. 1994. Carbon Mineralization. In: WeaverWRW et al., editors. Methods of soil analysis. Part 2.Microbiological and biochemical properties. Madison, WI.p 835-63.Buchmann N. 2000. Biotic and abiotic factors controllingsoil respiration rates in Picea abies stands. Soil Biologyand Biochemistry 32:1625-35.griwoldSpecialized equipment, shortcuts, tips:Draeger-TubesⓇ (fig. 1) contain chemical reagents thatchange color in the presence of CO2. The length of thecolor change indicates the measured concentration of CO2.Before using a Draeger-Tube®, check its expiration dateand always store them at the recommended temperature.Another popular assessment method is the SolvitaⓇ Soiltest. Mention of commercial products does not constitutean official endorsement by the U.S. Department ofAgriculture.Time needed: 30 minutesrelative soil respiration1.00.80.6-0.4-0.20.0Aubia10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100% water-filled pore spaceFigure 2. Soil respiration in relationship to water-filled pore spacAfter Parkin et al., 1996.

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One important measure of biological activity in the soil ecosystem is soil respiration. It is a…

View the figure 2 which shows the relationship between water filled pore space and relative soil respiration. What generalization can you make regarding the optimum soil water conditions for biological activity? Use the photos to answer the question figure 2 is in the photos