Land Reclamation Effects On Soil Nutrient Distribution in a Surface Mine Chronosequence in East Texas

Ng, J. P., Hons, F.M., and Gentry, T.J. Texas A&M University, College Station, TX.

A problem in reclaiming surface mines to native conditions is the poor soil quality thatimpedes revegetation in post-mined soils. Our objectives were to measure the chemicalcharacteristics distribution of nutrients in the soil profile to 1 m over a chronosequence of 40years to determine when a reclaimed soil returned to premined conditions. In addition, wecompared mine soils subjected to two different reclamation practices (crosspit spreader andmixed overburden) at 20 years of age at Big Brown lignite mine in East Texas. Soil qualityindicators, including soil organic carbon, were able to reach and exceed preminedconcentrations almost immediately after reclamation began, although the distributionthrough the soil profile required at least 5 years before stratification was observed. Nmaintained premined levels and profile distribution after 15 years of reclamation, while P wasable to stratify after 5 years but not reach premined conditions. Other nutrients were able toexceed premined conditions and develop native profile distribution 10-15 years (K and Ca),while Mg, S, and Fe displayed an increase in depth. When comparing the two reclamationpractices at 20 years of age, the crosspit spreader showed better stratification between 0-15and soil deeper than 15cm, but lower concentrations of nutrients compared to the mixedoverburden treatment. The stratification of soil nutrients in post-mined soils indicate a returnof biological activity, which is strongly influenced by the processes used to regrade thedisturbed landscape and the timing of revegetation. Changes in soil textural classificationappear to correlate with the increase of nutrients during site rehabilitation. We conclude thatthe more recent implementation of crosspit spreading served as a better method to returnmined soils to premined conditions, but more research should be conducted to determinehow aggregates and biological activity from the soil microbial community are influencing theaccumulation of soil organic carbon over surface mine reclamation age.

ABSTRACT

METHODOLOGY

RESULTS

CONCLUSIONS

• Sites had a uniform progression of age, minimal slope, and on top of hills to avoid accumulation of runoff and nutrient variation. • Five sites were selected with the CP reclamation treatment spanning 0 to 20 years: CP0, CP5, CP10, CP15, and CP20 • Three sites were selected with the older MO treatment aged 20 to 40 years: MO20, MO30, and MO40 • One unmined site was selected as a control: NP• Sampling occurred on June 23 and 24, 2009.• Vegetation: Coastal bermudagrass, Yuchi arrowleaf clover, and Crimson clover.

• Four transects 30-m long at each site• 10 1-m cores were taken along each transect• Each soil core was divided into 5 lengths: 0-5, 5-15, 15-45, 45-75, and 75-100 cm, and combined• SOC, TC, and TN analyzed by the combustion procedure• Chemi-thermal method was used to determine coal C and N content• Mehlich III and a DPTA solution were used for all other nutrients

Fig 1. Big Brown Mine is located near the town of Fairfield, TX.

Questions or Comments? Please contact me at justin.ng@tamu.edu We acknowledge Luminant for providing research funding and support.

OBJECTIVESDetermine how soil properties change over time by selecting sites of different ages that are presumed to have been treated the same.Determine the length of time required to achieve undisturbed (pre-mined) soil quality by focusing on multiple interrelated determinants of soil quality

Physical Properties• Texture varied greatly between sites, but uniform through depth

• Sandy in NP • Sandy loam and sandy clay loam in CP sites • Clay loam in MO sites • Axtell, Lufkin, Tabor series were predominant.

• Bulk Density• Reclaimed soils through 15 years had higher

BD than unmined sites.• BD increased with depth.• CP20 marked a return to NP levels.• All MO sites had lower BD than the NP.

Fig 2. Nine sites selected for the chronosequence in Freestone County, TX.

Fig 3. Operational dragline and crosspit spreader in an active surface mine.

Fig 4. Reclaimed 20-yr old mixed overburden mine site (MO20).

Coal Influences

Phosphorus and Potassium

CHEMICAL PROPERTIES

•Coal Carbon• Carbon increased with age

of reclamation.• CP coal C was uniform with

depth• MO coal C showed variability

and no trend with depth and had the highest percentages of C.

• More C was detected in MO v. CP.

• Coal Nitrogen• Nitrogen was higher in

reclaimed sites than NP.• There was an apparent trend

of increasing nitrogen with age.

• There wasn’t a strong trend of N with depth.

• More N was detected in MO compared to CP.

• Phosphorus• P in the NP site was higher than

all reclaimed sites.• Less stratification was observed

≥ 15 cm in reclaimed sites.• CP sites peaked at 15 yrs.• MO P levels were similar

between 20 and 40 yrs.• At 20 yrs, MO had higher P than

CP sites, but only in 0-15 cm.

• Potassium• K in the NP site was lower than

all reclaimed sites.• Stratification did not occur until

10 yrs after reclamation.• K levels were highest at 10

years in CP sites.• MO K levels were similar by

age.• At 20 yrs, MO had higher K than

CP sites.

• pH• Decreasing pH was observed in CP sites with

increasing age and depth except at CP0. • MO sites also had decreasing pH with depth, but

were similar with age.• CP soils were more acidic than MO soils.• Stratification of pH was observed after 5 yrs.

• Electrical Conductivity• EC in reclaimed sites increased with depth in

comparison to the NP site.• EC in soils lower than 15 cm was lower than

topsoil after 15 yrs.• In CP sites, EC decreased after reclamation

began.• MO EC was consistent in all sites.

• CEC• NP CEC was lower than CP reclamation sites• No apparent trend could be observed with age.• Stratification was not observed in CP sites.• Lignite CEC = 111.1 meq/100g.

FUTURE RESEARCH• Soil quality is composed of many interrelated factors, which must be investigated to determine how successful current reclamation practices are.• Taxonomic and functional microbiological data will be gathered and correlated with the changes in soil data.• Determine soil aggregation and the activity of each fraction. The finer textured soils found in reclaimed sites are probably related to increases in carbon. This future study will include measurements in soil respiration and soil microbial biomass.

• Carbon• Carbon in NP displayed a

decreasing trend with age.• In comparison to NP,

reclamation sites had higher a C% in 0-15 cm, but similar C% at lower depths.

• Variability was highest shortly after reclamation and ≥20 yrs.

• C% increased with age.

• Nitrogen• Nitrogen in NP displayed a

decreasing trend with age.• Not until CP15 does is this

trend observed.• Nitrogen at ≥20 yrs increases

compared to NP.

• Carbon to Nitrogen Ratio• At 0-15 cm, ratios stay

between 9.0 and 18.0 for all ages of reclamation.

Carbon and Nitrogen

Secondary Nutrients

• Calcium• Increased dramatically after

reclamation.• Stratification seemed more

consistent ≥20 yrs.• MO > CP with the exception of

Ca levels at CP0.• Magnesium

• Increased dramatically after reclamation.

• Mg levels appeared to increase with depth in all reclamation sites.

• Sulfur• S levels were higher after

reclamation compared to NP.• At 15 yrs of reclamation, S

levels in 0-15 cm decreased closer to NP levels.

• After reclamation, S increases with depth. A better trend with depth is observed at 10 years.

• Coal contributed to both C and N levels, but more in reclaimed soils. The presence of carbon and nitrogen in NP sites suggests a form of non-coal resistant organic material.• C/N ratios in 0-15 cm suggest a very active soil microbial community with net mineralization rates.• Nutrient stratification was observed after 10 years of reclamation. Better stratification was observed in CP sites compared to MO sites. This was probably due to variability from the older reclamation technique (MO), older plant growth, root penetration and nutrient uptake.• Nutrient concentrations were mostly higher in MO sites compared to CP sites.• pH decline over time is an area of concern as it may indicate the presence of acid-forming materials. However, it could also indicate an increase in soil respiration resulting in carbonic acid.• Although EC levels increased dramatically after reclamation, levels were lower than the threshold range considered detrimental to plant growth (> 1 mmhos/cm).