The effects of disturbance on soil characteristicsrelevant for revegetation
While dry land soils may appear lifeless much of the year,living organisms, from bacteria to animals and plants, stronglyinfluence their fertility, structure, and response todisturbance. Small organisms such as ants, bacteria, fungi,microarthropods, nematodes, earthworms, springtails, protozoans,termites, and yeasts play important roles in soil nutrientcycling and development. Many of these little-noticed organismsare easily disturbed or destroyed by human activities and theirelimination can lead to undesirable changes in soil moisturerelations, soil structure and fertility and plant and animalcommunities. Much more research has been conducted on aboveground plant architecture and characteristics than on the equallyimportant soil properties and soil ecosystems.
Some of the most important impacts of disturbance are the often unseen effects at and below ground level. Changes in physical, chemical and biological factors have been addressed in several studies and key changes observed include: reduce infiltration and fertility, increased compaction and soil strength, increase erosion and reduced biological activity. These factors can be assessed with an impact penetrometer, simple infiltrometers, and chemical and biological assays, such as mycorrhizal and bacterial infectivity tests.
Almost all types of disturbance degrade soil structure. One ofthe most serious problems is compaction caused by vehicles, tiredynamics, equipment operations, pedestrians, animals andbicycles. Grading and filling are the most destructive, but evenminimal activity can have significant adverse effects on soilstructure. Loamy soils are more sensitive to compaction thansandy soil, and wet soils are much more vulnerable than drysoils.
Disturbances can affect structure at a much greater depth thanmight be expected. In one test, significant adverse changes wereobserved at 10 inches depth from as little as three passes with afour wheel drive vehicle over moist soil. In some cases, soilstrength can be significantly increased by one pass, but morecommonly, the soil strength increases with repeated passes. Thesoil strength after 10 passes of a 4 wheel drive vehicle on onetest day exceeded three times the minimum amount causing seriousreduction in root growth.
The adverse effects of compaction on annuals as well asperennial plants have been studied. Compaction of a loamy desertsoil by off-road vehicles resulted in significant reductions indesert annuals in one Bureau of Land Management study. Theeffects of this type of compaction can be dramatic even infavorable environments. For example, equipment operation whichincreased soil bulk density (the density per volume) only 12% onlogging decks and skid roads, in a favorable forest environment,reduced seedling survival 90% (Lockably and Vidrine, 1988)
Compaction also affects the movement of water and air acrossthe soil surface boundary. Infiltration, the movement of waterinto the soils, is critical for plant and soil health. If watercan't move into the soil quickly, then it will pond and run off,leaving plants dry and dying, increasing erosion and increasingflood frequency and magnitude. Most of the increases in bulkdensity from compaction and disturbance appear to result from thedestruction of larger soil pores.
For simple infiltration studies a series of meter plastictubes are set into the soil in slots cut with a hole saw and thenfilled with water. Infiltration is measured by the fall of waterin the tube over time. For more detailed analysis, a double ringinfiltrometer, sprinkling infiltrometer or other types ofinstruments can be used.
A study of a creosote scrub plant community in Nevada clearlyillustrates the impact of vehicle operation on infiltration,Table 4-1, after Eckert et al. (1979).
Table 4-1. The Impact of off-road-vehicles on terminalinfiltration
|cm/hr||change from control|
Recovery from compaction is very slow in areas without winterfreezes, because no frost heaving occurs. Infiltration in aMojave desert vehicle parking lot used for military operationsfive decades earlier was only about half that of undisturbed areanearby. The removal of vegetation by disturbance can furtherreduce infiltration as plant-mediated infiltration benefits (stemflow, litter, etc.) are eliminated. Infiltration in dry creosotebush soil was double that of dry base soil, and infiltration inwet creosote bush was almost five times higher than in wet baresoil. Infiltration in ashy spike moss covered soils was muchfaster than in adjacent bare soils. During intense rains thesechanges in infiltration are accentuated. Areas with good plantcover may hold and save much of the rain that falls in intensestorms while areas that have been disturbed experience sheetflow, flash floods, and severe erosion.
Soil saturation percentage (SP), the amount of water that ittakes to saturate the soil expressed as a percentage of the soildry wet, is a useful indicator of soil texture and water andnutrient holding capacity. Sandy soils commonly have a low SP(18-24 percent), reflecting limited water holding capacity andlow fertility. The SP of surface soils under multi-stemmedcreosote bush and bur sage at a sandy CalTrans site in theCoachella Valley was much higher (31-37 percent) than that of thesoil between plants (19-24 percent), suggesting that the finesoil particles and organic matter accumulating beneath the plantcanopies improve the water and nutrient retention capacity ofsoils. In addition, higher long term soil moisture under desertshrubs has been observed in several studies, a marked contrast tomore humid areas.
Soils in the dry desert Southwest often have very low nutrientlevels. Low levels of macronutrients, nitrogen, phosphorous, andpotassium are generally not a problem unless the surface soilhave been lost, but micro nutrient imbalances may be important,Table 4.2.
Table 4.2. Soil chemistry at Red Rock Canyon State Park andScripps Knoll (n=number of samples)
|Red Rock Site type||% OM||ppm N03||PPM Pext||Ratio N/P||PPM Mg||PPM S||PPM Zn||PPM Mn||PPM Fe||PPM Cu||PPM B||pH||TKN PPM|
|Ant mound n=2||1.10||32.5||30||1.1||80||19||0.7||9.5||3||0.2||1.5||7.9||633|
|Less disturbed n=19||0.83||28.4||14.3||2.0||102||11||0.3||6.3||3.7||0.3||1.6||7.6||510|
|Very disturbed n=111||0.84||13.1||12.3||1.1||93||6.3||0.3||4.4||5.1||0.3||0.9||7.2||536|
|Scripps Knoll Site type||% OM||PPM N03||PPM Pext||Ratio N/P||PPM Mg||PPM S||PPM Zn||PPM MN||PPM Fe||PPM Cu||PPM B||pH||TKN PPM|
|Less disturbed n=4||6.45||21||20.8||1.0||336||19.0||4.3||14||31||0.6||1.0||5.5||2937|
|Very disturbed n=4||1.95||15||29.5||0.5||430||40.8||2.1||13||47||5.4||0.9||6.0||1407|
Phosphorus and nitrogen levels tend to be concentrated in thetop 2-3 cm of soil in dry land soils. Construction, erosion orsevere disturbance can dramatically reduce the already low soilfertility as wind and water erosion remove these more fertilesurface soils. These differences are shown in table 4-3 in AnzaBorrego Desert State Park. ON slopes as areas with extensiveerosion the nutrient levels may be dramatically reduced.
Table 4-3. Soil fertility as affected by increasingdisturbance. ABDSP
|PPM NO3||PPM Ext. P||Ratio NO3/P|
|Plant canopy, n=14||4.77||0.91||5.2|
|Between plants, n=9||1.59||0.72||2.2|
While a reduction in available nutrients is usually the problem, adding nutrients can also be problematic. Nitrogen added to the soil from pollution related dryfall can pose problems throughout the drylands of the Southwest. The deposition rates in the desert rarely approach the 30-40 kg ha found in urban areas in Southern California, but even the modest additions can increase surface nitrogen dramatically and shift the competitive balance to nonnative weeds. Studies at Fort Irwin showed a dramatic seasonal increase in surface nitrogen, apparently from deposition.
In the Antelope Valley, the most degraded areas had thehighest nitrate levels, Table 4.4. These probably reflects bothatmospheric deposition and fertilizer from farm operations. Sincemost weeds are more responsive to high nutrient levels thannative plants the survival or establishment of native plants canbe adversely affected. These higher soil nitrogen levels alsoincrease grazing pressure by making the plants more tasty forherbivores. The ratio of nitrogen to phosphorus may also beimportant because it can affect the root to shoot ratio. Excessnitrogen can increase shoot growth, increasing moisture stress.
Table 4-4. Surface soils in the Antelope Valley, 0-10 cm.
Variation was high for some sites, means of 3 samples, ug/g
|Site||NH4 PPM||No3 ppm||Ratio||Site condition|
|AV Site K||1.62||4.16||2.6||second worst site|
|AV Site B||0.79||0.41||0.5||moderate|
|AV Site W||0.22||0.35||1.5||moderate|
|AV B120 W||0.66||0.60||0.9||moderate|
|AV C100 N||1.92||1.54||0.8||best|
Understanding cumulative effects
If the various effects of disturbance are consideredseparately, a single change may not appear to be verysignificant. Following Leibig's discovery of the importance ofmacronutrients (N,P,K) in the 1800's, it was felt that the mostdeficient element would be the limiting factor. But as increasingknowledge of plant biochemistry and nutrition has emerged, it hasbecome clear that the problem of deficiency is more complex andreflects an interaction among multiple factors. Micronutrients,which affect many plant hormone and growth regulators, areparticularly important. The cumulative effect of disturbance ismore likely to reflect an interaction of all the deficiencies.Applying this approach to the Red Rock and Scripps Knoll soilsresults in the following equation, Table 4.5.
Table 4-5. The cumulative effect of disturbance
The ratio of disturbed/less disturbed
For both sides, the regeneration potential calculated from thecumulative effect of disturbance may be only two percent of lessdisturbed soils. It is likely that the effects of soil nutrientproblems alone would not be this severe, because if we fullyunderstood the interactions between nutrients we might find thatsome are not that critical. But the soil nutrient problems areaccentuated by problems in soil structure. For these disturbedsites it is not uncommon to find soil infiltration is cut in halfand soils strength, particularly in the critical surface layers,may be double less disturbed soils. If we add these factors intothe disturbance equation, we find:
Soil chemistry * soil infiltration * soil strength = Netregeneration potential
For these two sites the calculated net effect would be:
Soil chemistry (0.02) * soil infiltration (0.5) *soil strength(0.3) =0.003
This suggests that the disturbed sites may have a regenerationpotential as low as 0.3% of less disturbed sites, not consideringsoil ecosystem health. Soil biological and ecological health alsomatters.
Soil ecological health
Compaction and disturbance can also reduce populations ofbeneficial soil organisms. Total numbers of fungi, bacteria,nematodes, microarthropods, and macroarthropods tend to be muchlower in disturbed soils. In addition plant pathogens are morecommon in disturbed soils. The effects of vehicle operation onants and termites have not been extensively studied but areprobably also important and reduction in ants and termitesadversely affects infiltration, aeration and nutrientavailability.
The changes in soil moisture and aeration in disturbed anddegraded soils caused reduced infiltration and lower moistureholding capacity adversely affect nodulation by changes in soilstructure and the elimination of animals and lizards that burrowin the soil can limit the movement of beneficial inocula in thesoil. Living or recently living roots and organisms can beassayed with Europium stains to asses soil health. Again, forAntelope Valley soils, the most degraded site had very lowbacterial and fungal activity, table 4-6.
Table 4-6. Soil data Antelope Valley, per gram of soil, means
|site||meters hyphae||bacteria -millions||Condition|
|AV Site K||0.3||87||second worst site|
|AV Site B||0.6||88||moderate|
|AV Site W||0.3||94||moderate|
|AV B120 W||0.7||92||moderate|
|AV C100 N||1.3||94||best|
Variation was very high for some sites, the hyphae meanexcludes the highest value
Mycorrhizal and rhizobial assessments can be made by growinglegumes or mycotrophic plants in site soils and examining rootinfection rates. Unfortunately these biological assessments,which are very informative, are time consuming and rarely done.
The cumulative effects of disturbance are striking, even ifonly modest degradation occurs in each component. For example, ifwe combine the Scripps Knoll data for infiltration, soil strengthand soil chemistry we find that the disturbed areas may have arevegetation potential of only: *****
In reality, this calculation probably exaggerates the problem,but it is clear that the revegetation potential of thesedisturbed site is only a fraction of the undisturbed or lessdisturbed areas nearby. That is why natural regeneration takeshundreds or thousands of years and explains why restoration iscostly and challenging unless site soil characteristics arerestored to predisturbance conditions.
Soil sampling to understand disturbance
The appropriate treatment for restoring soils on a given sitecan only be determined by sampling. The type of soil samplingthat is done is usually based on budget and the soil sampler'sexperience and training. Standard soil tests for nitrogen andphosphorous are rarely useful, because test labs are usuallylooking for agricultural, not native plant community, soilcharacteristics. Good testing requires the establishment of areference "undisturbed" or native soil. Micronutrients,pH, and organic matter are likely to be more important thannitrogen and phosphorous. The carbon:nitrogen ratio of theorganic matter may also be important.
The following should be considered in soil sampling:
1. Take sufficient samples
The variability in soils is usually very high, so it takesmany samples, both near plants (properties can vary depending onspecies) and in open areas between plants, to get good data. Theproperties of soils at different depths may also be important,even up to 3-10 feet or more in some cases. The differencebetween nitrogen in the top 1/2-3/4 inch and deeper soils canhelp reveal the impact of particulate dryfall.
Samples can be combined, bulked, mixed thoroughly andsubsampled by eye or with a splitter box to reduce the cost oftesting. Samples for most biological studies must berefrigerated, while samples for soil chemistry and texture areusually air dried. A useful multi-nutrient soil test may cost $50per sample. A sample sheet from A&L Western Agricultural Labsis shown at the end of this report.
2. Examine physical properties
Infiltration and soils strength are very important. These canstudied with an impact penetrometer and various types ofinfiltrometers. Pore size can also be measured, but this is morecomplex.
3. Examine biological properties
Soil ecology can be assessed with infectivity studies, Europium staining or growth studies. A growth test is always useful to make sure no toxic material are present in the soil. This might be boron herbicides that would not be detected in most soil analyses. Boron was historically used as a weed control agent and is likely to appear on old industrial sites and some farms. Levels of boron above 0.5 PPM can be injurious to plant health.
A count of ant mounts and burrows can be useful indicators ofecosystem health. Ants are as important in dryland soils asearthworms are in moister areas and are important in therestoration process. There are also two invasive ants of concern.The aggressive but small Argentine ant is invading much ofSouthern California and is often spread with transplants. Fireants are also making their way into the Southwest.
Soil cover and litter depth on undisturbed sites can berelevant and are easy to sample.
4. Examine chemical properties.
The standard tests for nitrogen and phosphorus are not veryuseful, and often not important, because native plants oftenbenefit from very low nutrient levels (by agriculturalstandards). The ration between nitrate and available phosphorusmay be important, and excess levels of nitrogen may be detected.If high nitrogen levels are detected, a slow to break downorganic material may be applied to tie up excess nitrogen. Thecarbon nitrogen ratios of healthy soils are typically 25:1 whilethe ratio in agricultural and disturbed soils may drop below10:1. Micronutrients and trace elements can be important but arerarely studied. Zinc deficiency appears to be common in SouthernCalifornia soils (see following paper).
Soil salinity and pH are likely to be important, andfortunately testing is inexpensive and fast. Soil organic matteris relatively easy to test (by combustion) and can be a usefulindicator of soil health. Depressed levels of organic matteraffect water holding, aeration and nutrient availability. Agrowth test should also be done to detect possibly high levels ofherbicides, boron or other toxic materials. Grow several speciesof grass and vegetables for a quick test.
Eckhert, R.E., M.K. Wood, W.H. Blackburn, and E.F. Peterson.1979. Impacts of off road vehicles on infiltration and sedimentproduction of two desert soils. Journal of Range Management32(5):394-397.
Lockaby, B.G and C.G. Vidrine. 1984. Effect of loggingequipment traffic on soil density and growth and survival ofyoung loblolly pine. Southern Journal of Applied Forestry8(2):109-112.
Probable zinc deficiency on disturbed sites
Zinc is one of the essential micronutrients for plants (Fe, MN, Cu, B, Mo, Cl are others). Zinc is important as an enzyme catalyst, and lack of zinc is often reflected in poor carbohydrate metabolism and protein synthesis. Zinc also stabilizes the association between catalytic and regulatory units of enzymes (Jenny, 1980). This can cause stunting because inadequate auxins (specifically tryptophan and its successor indoleatic acid, IAA) may be produced (Landis and van Steenis, 1988). This may be revealed in reduced internode elongation or stunted foliage, in some cases resulting in rosetting and little leaf syndrome. Chlorosis of younger leaves with interveinal mottling is diagnostic in citrus, but foliar symptoms in other plants are more variable and little known about the response of native plants.
Zinc deficiency is not uncommon and is most important in younger tissues. This may make it very important in early growth and plant establishment. Inadequate zinc may reduce growth of roots below critical levels as a result of depressed metabolism. Zinc deficiency may also result in increased susceptibility to disease. In some agricultural crops, zinc deficiency prevents flowers from forming, and in others, seeds may fail to develop properly (Parnes, 1990). Zinc can be measured in the soils or in plant tissues. Tissue testing is often done in agricultural crops, but no levels of desirable levels of zinc in native plants are available. Zinc deficiency may be caused by the inadequate zinc in the soil or low availability of this essential nutrient. The level of zinc needed for healthy coastal sage scrub is unknown, but for seedlings of many trees it is fairly high. For example, the minimum desirable tissue levels for zinc in a study of Monterey pine (Pinus radiata) seedlings was 11 PPM (McGrath and Robson, 1984). Most agricultural crops respond to added zinc when level drops below 0.5 PPM DTPA extractable zinc (Brown and de Boer, 1983), and levels below 1.5 PPM are of concern (Soil Improvement Committee, 1998).
Plants concentrate zinc and other organic matters generallyhas much higher levels of available zinc than surrounding soils.Organic matter in the soil may also be required to maintain zincin a chelated or available form (Parnes, 1990). Low organicmatter in soils may be a contributing factor in zinc deficiency.Zinc is more available in acidic soils and less available inbasic soils (Pierzynski et al., 1993). Zinc uptake is facilitatedby the beneficial association mycorrhizal fungi form with plantroots (Landis and van Steenis, 1998). Zinc availability alsoincreases in more aerated soils (Parnes, 1990).
Few studies have been done to determine normal levels of available or total zinc in Southern California soils, but where studies have been done a range of 25-200 PPM total zinc is considered normal (Bowie and Thornton, 1985). The available zinc in 461 California soils averaged 1.23 PPM, with a high of 10.4 PPM (Brown and de Boer, 1983). In our testing of 105 samples soils that were probably zinc deficient (<1.5 PPM) or deficient (<0.5 PPM available zinc) were very common, Table 1.
Table 1. Possible zinc deficiency
|n||Percent 0.5 PPM or less||Percent 1.5 PPM or less|
|Red Rock Canyon||42||90||98|
The low zinc levels in disturbed and very disturbed soils inSouthern California are compounded by increased soil pH, soilcompaction and decreased organic matter. If we assume a directlinkage between zinc and organic matter the disturbed soils haveonly one fourth the available zinc, and very disturbed soils haveonly one tenth the available zinc of less disturbed soils. Addingin compaction of loss of soil symbionts zinc availability indegraded soils may sink as low as 5% of the level in lessdisturbed soils.
We add more zinc or make zinc more available by adding organic matter. This adds small amounts of zinc and increases zinc availability. Using acidic compost, such as pine needles, reduces soil pH and further increases zinc availability. Reducing soils compaction by ripping or spading improve aeration and facilitate growth of mycorrhizal simbionts. Zinc can also be added by using organic fertilizers, like poultry manure, that have more zinc. In extreme cases a zinc fertilizer, such as zinc sulfate which is 25-35% zinc, can be used (Parnes, 1990). A typical application rate would be 2-20 pounds per acre. Zinc can also be applied in a chelated micronutrient fertilizer, either as a dry powder added to the soil or sprayed as a foliar feed (Peaceful Valley, 1999). We will evaluate zinc fertilization during 1999-2000.
Bowie, S.H.U. and I.Thornton, eds. 1985. EnvironmentalGeochemistry and Health. Kluwer, Hingham, M.A.
Brown, A.L. and G.J. de Boer. 1983. Soil tests for zinc, iron, manganese, and copper. Pp. 41-42. In H.M. Reisnauer, ed. Soil and Plant Tissue Testing in California. U. C. Division of Agricultural Sciences, Berkeley, CA. Bulletin 1879.
Henny, H. 1980. The Soil Resource. Springer Verlag, NY. 377p.
Landis, T. and E. van Steenis. 1998. Micronutrients-Zinc.Forestry Notes, July. 10-14.
McGrath, J.F. And A.D. Robson. 1984. The distribution of Zn and the diagnosis of Zn deficiency in seedlings of Pinus radiata. Australian Forest Research 14(3):175-184.
Parnes, R. 1990. Fertile Soil: A Growers Guide to Organic andInorganic Fertilizers. AgAcces, Davis CA. 190p.
Peaceful Valley Farm Supply. 1999. Tools and Supplies forOrganic Farmers and Gardeners. Grass Valley, CA.
Pierznski, G.M., J.T. Sims and G.F. Vance. 1993. Soils andEnvironmental Quality. Lewis, Boca Raton, FL. 313p.
Soil Improvement Committee. 1998. Western Fertilizer Handbook.California Fertilizer Association, Interstate Publishers,Danville, IL. 362 p.
Soil test data for comparisons