Soil solution nitrate-N concentrations at 0.3 m below soil surface were less in silvopastures than open pasture or conventionally thinned pine plantation at various time periods during this study. Fertilization with N resulted in time-delayed spikes in soil solution nitrate-N concentrations. At 1.2 m below soil surface, soil solution nitrate-N concentrations were less in the silvopastures and the open pasture than in the thinned pine plantation only during winter months. The results partially corroborate our hypothesis of reduced nitrate-N leaching in silvopastures compared with the other investigated production systems. Fertilization rates designed for forage production generally improved tree nutritional status.
Silvopasture is an intentional integration of trees, improved forages and livestock into one management system (Sharrow 1999, Sharrow and Ismail 2004). Formerly known as tree-pasture or pine-pasture, silvopasture is one of several agroforestry practices steadily gaining popularity in the southeastern U.S. (Bendfeldt et al. 2001, Grado et al. 2001, Husak and Grado 2002, Workman et al. 2003). Well-managed silvopasture can improve cash flow of small family farm operations (e.g. Dangerfield and Harwell 1990) and provide many environmental benefits (Nowak and Blount 2002). Silvopastures are usually established by planting trees in existing pastures with enough open space left between the rows of trees to allow for forage production (Grado et al. 2001, Husak and Grado 2002). However, these tree-forage-livestock systems can also be established after commercial thinnings of pine plantations (Clason 1995, Clason 1999). Abundance of mid-rotation age pine plantations in the South creates ample opportunity for such conversions to take place.
Many non-industrial private forest landowners may adopt alternative forest-based enterprises if they are convinced of their profitability and environmental qualities (Alavalapati et al. 2004). To test the feasibility and environmental benefits of mid-rotation pine plantation conversion to silvopasture in North Florida, an 18-year-old loblolly pine (Pinus taeda L.) stand was thinned to two silvopastoral tree configurations while the control was conventionally 5th row thinned. Warm- and cool-season forages were established in both the silvopastoral systems and on a nearby open pasture. These tree and pasture systems are subject of a doctoral research project of Susan Bambo, who is a graduate student at the University of Florida in the School of Forest Resources and Conservation. One of the hypotheses tested in this project was that silvopastoral systems reduce soil nitrate-N leaching compared with open pastures and pine plantations. Recently, Allen et al. (2004) found reduced soil nitrate nitrogen leaching in a pecan (Carya illinoensis K. Koch) – cotton (Gossypium hirsutum L.) alley cropping system, but to our knowledge, similar environmental benefits of silvopasture have not yet been documented. In addition, we tested a hypothesis that fertilization for forage production improves tree nutritional status in silvopastoral systems.
- Determine differences in soil nitrate-N leaching among silvopasture, conventionally thinned loblolly pine plantation and open pasture.
Determine effects of silvopasture fertilization for forage production on tree nutrition.
The project was conducted on a private farm in Gadsden County located in panhandle region of north Florida. This area is located at latitude 30.55 dgrees North and longitude 84.60 degrees West, with an average elevation of 61 m (200 ft) above see level. The climate is temperate with moderate winters and hot, humid summers. Annual rainfall averages 1676 mm (66 in), and the average daily minimum and maximum air temperatures are 12.8 and 26.1 oC (55 and 79 oF), with an average temperature of 15.6 oC (60 oF). The soils in the study site area are loamy fine sands (fine-loamy, kaolinitic, thermic Typic Kandiudults).
An 18-year-old loblolly pine plantation was thinned to two silvopastoral tree spacings before forage establishment, while control was conventionally 5th row thinned. The two silvopastoral tree configurations tested were: (1) heavy conventional 4th row thinning, and (2) double tree row with forage alleys. In double row configuration, trees were spaced 3.0 x 3.7 m (10 x 12 ft) and were separated by 15.2 m (50 ft) wide forage alley between each pair of double-row tree sets. The double tree row configuration was “an after thinning” version of the popular in Florida 1.2 x 2.4 x 12.2 m (4 x 8 x 40 ft) tree spacing used for silvopasture establishment ever since its inception (Lewis et al. 1985). Both silvopastoral treatments were thinned to approximately 200 trees per hectare (80 trees per acre, or TPA) and 6.9 square meter (30 square feet) of basal area (BA), whereas the control was thinned to approximately 400 trees per hectare (160 TPA) and 13.8 square meter or 60 BA. The response variables for the larger, separately funded project, were tree height, diameter, crown width and length, and tree and stand wood volume.
Argentine bahiagrass (Paspalum notatum Flugge), Jumbo ryegrass (Lolium multiflorum Lam.), Dixie crimson clover (Trifolium incarnatem L.), and Cherokee red clover (Trifolium pretense L.) were identified for this project based on their relative shade and drought tolerance. These forage varieties showed good performance under varied photoperiod in previous research (Blount, personal communication). Forage combinations were assigned at random to four replicated 18.3 x 20.1 m (60 x 66 ft) plots within each silvopasture and a nearby open pasture. The following combinations were used: (1) bahiagrass alone, (2) bahiagrass plus ryegrass, (3) bahiagrass plus ryegrass plus crimson clover, (4) bahiagrass plus ryegrass plus crimson clover plus red clover. Bahiagrass was seeded in all forage plots at 22.4 kg/ha (20 lbs/acre) in July 2003, and winter forages were overseeded in November 2003. Seeding rate was 15.7 kg/ha (14 lbs/acre) for ryegrass and red clover, and 16.8 kg/ha (15 lbs/acre) for crimson clover. The appropriate Rhizobium trifolii inoculum was applied to crimson clover seeds immediately prior to seeding. The red clover seeds were inoculated by seed distributor. Site preparation before forage establishment included prescribed fire to remove logging debris, and herbicide (glyphosate) to control herbaceous weeds. Residual sweetgum and cherry trees were cut down and the stumps treated with herbicide (triclopyr) to prevent sprouting. The area was disk-harrowed prior to seeding bahiagrass. Bahiagrass was mowed and plots cultipacked after overseeding winter forages. The response variables of the larger, separately funded project were forage yield per hectare (acre) and forage quality (crude protein content, in vitro organic matter digestibility).
Fertilization treatments were based on soil testing results and forage needs. In 2004, bahiagrass plots in the open pasture received (rates in kg of element per hectare, in parentheses in lbs of element per acre) 168.3 (150) N and 129.0 (115) K; bahiagrass plots in the silvopastures 168.3 (150) N, 44.9 (40) P and 163.8 (146) K; and the conventionally 5th row thinned pine plantation plots 168.3 (150) N, 44.9 (40) P and 167.2 (149) K. Winter forage plots in the open pasture received 112.2 (100) N and 65.1 (58) K; and winter forage plots in silvopastures 112.2 (100) N, 44.9 (40) P and 154.8 (138) K. These varied fertilization regimes were meant to equalize the soil nutritional status from the standpoint of forage growth. After the funding for this sub-project commenced on September 1, 2004, all bahiagrass and winter forages, and thinned pine plantation plots received 168.3 (150) N, 39.3 (35) K, 11.2 (10) Mg and 22.4 (20) S in the following fashion. All winter forage plots received 56.1 (50) N on December 28, 2004 and 56.1 (50) N on March 14, 2005, the remainder consisting of 56.1 (50) N, 39.3 (35) K, 11.2 (10) Mg and 22.4 (20) S was applied on July 1, 2005. All bahiagrass forage plots and thinned pine plantation plots received 112.2 (100) N on April 21, 2005 and the remainder consisting of 56.1 (50) N plus 39.3 (35) K, 11.2 (10) Mg and 22.4 (20) S on July 1, 2005. The following mineral fertilizers alone or in appropriate mixtures with each other were used to achieve desired amounts of elemental fertilizer applied on each date: ammonium nitrate (NH4NO3), ammonium sulfate [(NH4)2SO4], urea [CO(NH2)2], triple superphosphate (TSP), muriate of potash (KCl), K-Mag [K2Mg2(SO4)]. For the sub-project, funded by this graduate student grant, the response variables were N, P, K, Ca and Mg concentrations in tree foliage in the two silvopastures and the thinned pine plantation.
Soil Solution Nitrate-N Monitoring
Soil solution nitrate-N was monitored in silvopastures, open pasture and conventionally 5th row thinned pine plantation. In the silvopastures, only bahiagrass forage plots were subjected to nitrate-N monitoring to avoid compounding effects associated with nitrogen fixation by leguminous species (both clovers employed in this experiment). Total of 64 porous ceramic cup suction lysimeters (Soil Moisture Equipment, Santa Barbara, CA) were installed in pairs at 0.3 m (1 ft) and 1.2 m (4 ft) below soil surface. There were four lysimeters (two at each depth) installed in the middle of each monitored plot. Soil solution sampling began before bahiagrass fertilization treatments in 2004, and continued at roughly monthly intervals throughout 2005. Soil solution samples were collected into 20 ml scintillation vials 48 to 72 hours after vacuum (30 to 50 kPa) was applied to lysimeters. The solution samples were acidified until analysis for nitrate-N by the Analytical Research Laboratory at the University of Florida in Gainesville. Soil solution nitrate-N concentration was used as an index of the amount of inorganic nitrogen produced in the soil in excess of tree and forage demand, and was the response variable in the sub-project funded by this grant.
To establish tree nutritional status foliage from a total of 96 trees was sampled and analyzed for five major macronutrients in January 2005. Needles from the first flush of the previous growing season were collected from the primary branches in the upper 1/3 of the crowns on the south side of each of 8 trees per thinning treatment plot (equivalent to 32 trees per thinning treatment). Before commencement of funding for this sub-project on September 1, 2004, only two trees per thinning treatment plot (total of 24 trees) were sampled in February 2004 in the same fashion as previously described. All foliage samples were oven-dried (70 oC) to a constant mass, ground to pass a 1 mm screen, and analyzed for N, P, K, Ca, and Mg.
The experiment was analyzed as a modified split plot design by the general linear procedures of SAS statistical package (SAS Institute, Inc., Cary, NC). Four tree canopy plots within each thinning treatment and within the open pasture treatment served as the main plots. Soil solution sampling depths served as the sub-plots. Tree foliage nutrient concentrations for each element were analyzed with replication within thinning treatment serving as an error term for the main plot effects.
In March 2004, prior to fertilization we found higher soil solution nitrate-N concentrations at 0.3 m below the soil surface in open pasture (6.2 mg/l) than the silvopastures (1.3 mg/l) and the conventionally thinned pine plantation (1.0 mg/l). On June 16, 2004, two and a half months after the first fertilization of warm-season forages in silvopastures and fertilization in pine plantation, we detected higher soil solution nitrate-N concentrations at 0.3 m below the soil surface in the conventionally thinned pine plantation than any other production system. The nitrate-N concentration for this thinning treatment depth combination peaked at 91.0 mg/l on September 3, 2004. The differences in soil solution nitrate-N concentrations between the pine plantation and the other systems remained significant through the remainder of 2004. On two sampling dates in 2004 (June 16 and December 9), also at 1.2 m below the soil surface, soil solution nitrate-N concentrations were higher (reaching 12.0 mg/l) in the conventionally thinned pine plantation than any other investigated production system.
Similarly to the 2004 results, the 2005 lysimeter sampling revealed peaks in soil solution nitrate-N concentrations measured at 0.3 m below the soil surface in all four production systems following both N fertilization events. The early season (April 21) fertilization treatment (112.2 kg N/ha) significantly increased nitrate-N concentrations in open pasture at 0.3 m below the soil surface for three consecutive sampling dates between May 2 and June 27. The highest recorded nitrate-N concentration was 294.4 mg/l in the same treatments on May 28, or 37 days after the first fertilization event. In the conventionally thinned pine plantation, the nitrate-N concentrations reached their highest value (222.6 mg/l) on the same date. The late season (July 1) fertilization treatment (56.1 kg N/ha) resulted in lower nitrate-N concentration peaks measured at 0.3 m below soil surface in all four investigated production systems. The nitrate-N concentrations measured at 0.3 m below the soil surface in the 5th row thinned pine plantation were significantly higher (reaching 123.6 mg/l) than those for the other production systems between September 27 and December 13, 2005 sampling dates. Although nitrate-N concentrations at 1.2 m below the soil surface were raised after the fertilization events in the open pasture (up to 18.4 mg/l) and the 5th row thinned pine plantation (37.1 mg/l), they were not significantly different from those in the silvopastoral treatments. For the 1.2 m sampling depth, only the concentrations measured between January 8 and April 3 in the 5th row thinned pine plantation were significantly higher (reaching 17.5 mg/l) than those in the other production systems.
Foliage chemical analyses revealed that prior to fertilization, tree foliar N in double tree row silvopastoral system (1.16 %) was slightly below the minimum recommended for loblolly pine (1.20 %). Foliar N in heavily 4th row thinned silvopasture and conventionally 5th row thinned pine plantation were adequate, but close to the minimum recommended. Also the other measured nutrients were at or above the minimums recommended. Only foliar K concentrations were almost twice as high (average of 0.54 %) as the minimum recommended for loblolly pine (0.30 %) by Jokela (2004).
From 2004 to 2005, fertilization treatments applied for forage production in the silvopastures increased tree needle N concentrations to 1.50 %. During the same time period, there was a similar increase of foliar N concentrations in trees grown in the fertilized 5th row thinned pine plantation, while concentrations of P and Ca remained largely the same between the sampling periods. From 2004 to 2005, foliar K concentrations decreased to an average of 0.40 % in all three pine containing production systems. Despite those changes, at the beginning of 2005, foliar nutrient concentrations were higher than the minimums recommended for loblolly pine for all five measured macronutrients in all three pine containing production systems. However, neither of the investigated systems differed significantly from any other with respect to tree nutritional status in 2004 or 2005.
Educational & Outreach Activities
Bambo, S. (Ph.D. dissertation in progress). Loblolly pine growth and forage performance under thinned tree canopies in North Florida. School of Forest Resources and Conservation, University of Florida.
Bambo, S., Nowak, J.; Blount, A.; Osiecka, A. and Myer, R. 2004. Loblolly pine growth and warm/cool-season forage performance under thinned tree canopies in North Florida. Abstracts. 1st World Congress of Agroforestry, Orlando, FL, June 27-July 2, 2004.
Bambo, S.; Nowak, J.; Blount, A.; Osiecka, A. and Myer, R. 2004. Loblolly pine growth and forage performance under thinned tree canopies in North Florida. Poster, 1st World Congress of Agroforestry, University of Florida, IFAS, Orlando, FL, June 27 – July 2, 2004.
Nowak, J.; Bambo, S.; Blount, A.; Osiecka, A. and Myer, R. 2004. From a mid-rotation loblolly pine plantation to silvopasture – a North Florida case study. Pre-conference Workshop Presentation, 1st World Congress of Agroforestry, University of Florida, IFAS, Orlando, FL, June 27 – July 2, 2004.
The first year results only partially corroborated our hypothesis that silvopastures reduce soil nitrate-N leaching compared with conventionally thinned and fertilized pine plantation. We were unable to demonstrate the same benefit of reduced nitrate-N leaching in silvopastures compared with open pastures. The second year results indicate that there might be a potential for nitrate-N leaching below the 1.2 m soil depth in open pasture (highest nitrate-N was 18.4 mg/l) and the 5th row thinned pine plantation (highest nitrate-N was 37.1 mg/l). Generally higher nitrate-N values at both 0.3 and 1.2 m below the soil surface in the second year (2005) of the experiment compared with the first (2004) may perhaps be a result of repeated fertilization events delivering to soil excessive amounts of nitrate-N and ammonium-N, which can be converted to nitrate-N via nitrification. Such results would indicate a need for very careful calibration of N fertilization in accordance to forage and pine plantation needs to limit the potential for nitrate-N leaching, especially from conventionally 5th row thinned pine plantations and open pastures. On the other hand, our results also indicate a need to fertilize silvopasture to maintain adequate tree foliar nutrient levels. This is illustrated by borderline tree foliar macronutrient concentrations (except K) in 2004, and their increases from 2004 to 2005. Such results further suggest that fertilizer amounts, kind of materials used and timing need to be carefully considered to avoid potential nitrate-N leaching while working toward increasing forage and timber yields.
Areas needing additional study
Further experimentation with amounts, kinds of fertilizer materials and timing to refine silvopasture fertilization regimes with the objective of meeting forage and tree nutritional needs without triggering nitrate-N leaching below tree root zone.
Further experimentation with tree and forage species combinations to optimize wood and forage production under different tree canopy regimes.
Experimentation with different livestock in the tested tree-forage systems to achieve optimal and fully integrated tree-forage-livestock silvopastoral systems.
Alavalapati, J.R.R.; Shrestha, R.K.; Stainback, G.A. and Matta, J.R. 2004. Agroforestry development: An environmental economic perspective. Agroforestry Systems. 61: 299-310.
Allen, S.C.; Jose, S.; Nair, P.K.R.; Brecke, B.J.; Nkedi-Kizza, P. and Ramsey, C.L. 2004. Safety-net role of tree roots: evidence from a pecan (Carya illinoensis K. Koch) – cotton (Gossypium hirsutum L.) alley cropping system in the southern United States. Forest Ecology and Management. 192: 395-407.
Bendfeldt, E.S.; Feldhake, C.M. and Burger, J.A. 2001. Establishing trees in an Appalachian silvopasture: response to shelters, grass control, mulch, and fertilization. Agroforestry Systems. 53: 291-295.
Clason, T.R. 1995. Economic implications of silvipastures on southern pine plantations. Agroforestry Systems. 29: 227-238.
Clason, T. R. 1999. Silvopastoral practices sustain timber and forage production in commercial loblolly pine plantations of northwest Louisiana, USA. Agroforestry Systems. 44: 293-303.
Dangerfield, C.W. and Harwell, R.L. 1990. An analysis of a silvopastoral system for the marginal land in the southeast United States. Agroforestry Systems. 10: 187-197.
Grado, S.C.; Hovermale, C.H. and St. Louis D.G. 2001. A financial analysis of silvopasture system in southern Mississippi. Agroforestry Systems, 53: 313-322.
Husak, A.L. and Grado, S.C. 2002. Monetary benefits in a southern silvopastoral system. Southern Journal of Applied Forestry. 26: 159-164.
Jokela, E.J. 2004. Nutrient Management of Southern Pines. In: Dickens, E.D.; Barnett, J.P.; Hubbard, W.G.; Jokela, E.J., eds. 2004. Slash pine: still growing and growing! Proceedings of the slash pine symposium. Gen. Tech. Rep. SRS-76. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research Station. Pp: 27 – 35.
Lewis, C.E.; Tanner, G.W.; Terry, W.S. 1985. Double vs. single-row pine plantations for wood and forage production. Southern Journal of Applied Forestry. 9(1): 55-61.
Nowak, J. and Blount, A. 2002. Silvopasture: more than cattle grazing in pine plantations. Forest Landowner. 61: 10-13.
Sharrow, S.H. 1999. Silvopastoralism: competition and facilitation between trees, livestock and improved grass-clover pastures on temperate rainfed lands. In: Agroforestry in Sustainable Agricultural Systems. L.E. Buck, J.P. Lassoie and E.C.M. Fernandes Eds. CRC Press LLC. Boca Raton, FL. 416p.
Sharrow, S.H. and Ismail, S. 2004. Carbon and nitrogen storage in agroforests, tree plantations, and pastures in western Oregon, USA. Agroforestry Systems. 60: 123-130.
Workman, S.W.; Bannister, M.E. and Nair, P.K.R. 2003. Agroforestry potential in the southeastern United States: perceptions of landowners and extension professionals. Agroforestry Systems. 59: 73-83.