Advanced cropping system for sustainable production of Fraser fir (Abies Fraseri) in Christmas tree productions
The present study investigated the effects of groundcover management on soil properties, nutrient leaching, weed control and Fraser fir performances. Cover cropping did slightly affected soil organic carbon concentration. However, a significant increase of soil total nitrogen (N) were associated with cover cropping compared to the controls. Tree performances were significantly lower in plots where cover crops were grown in continuous patch. Clover and rye grass were more effective at suppessing weed populations than alfalfa as indicated by weed biomass yields. Significantly higher concentrations of total N and K were found in leachate from the controls than the cover crop treatments.
The specific objectives of the research were to:
1) Investigate the effect of the cover crop on soil physical and chemical properties and tree growth;
2) Evaluate the effectiveness of the cover crop in weed control;
3) Evaluate the effectiveness of integrated systems to chemical leaching.
Objective#1: Investigate the effect of the cover crop on soil physical and chemical properties and tree growth
1.1. Soil physical and chemical properties
Treatment means for soil pH ranged from 5.7 to 6.3 but were not significantly different (P>0.05) within the three sampling depths. The average soil moisture content varied between 18.2% and 21.9% for the 0 to 15 cm sampling depth, 17.0% and 19.1% for the 15 – 30cm sampling depth, and 14.0% and 17.8% for the 30 – 45 cm sampling depth. Across all treatments, soil moisture at the 0-15 cm, the 15-30 cm and the 30-45 cm averaged 20.1%, 18.6% and 15.4%, respectively. There was no significant effect of groundcover management treatments on soil moisture at any of the soil depths (P>0.05). However, soil moisture content significantly decreased with soil depths (P<0.001). Soil moisture was significantly lower at the 30-45 cm depth, but no significant difference was found between 0-15 cm and 15-30 cm depths.
Soil bulk density was not statistically different at any of the soil depths (P>0.05). Although the control had the lowest mean soil total C concentration, it was not significantly different (P > 0.05) from any of the other treatment means. No specific trend was observed between treatments at the 15-30 cm and 30-45cm depths. There was a significant decrease in soil total carbon with soil depth (P<0.001), most likely due to decreases in plant-derived C in deeper soil layers.
Total soil N was significantly higher in soils under cover crop treatments compared to conventional control plots at the 0-15 cm depth. Similar to organic carbon, there was no statistical difference of organic N in deeper soil core specimens. The C:N ratio values followed the same trend, generally decreasing significantly with soil depth (P0.05).
1.2. Tree performances
a) Tree survival rates
Tree survival rate showed significant differences (p<0.05) in 2007 (data not shown) and highly significant differences in 2008 (p0.05). The highest tree mortality was recorded on white clover plots without banding, followed by perennial rye grass and alfalfa. The rate of dead trees shown to be more severe in Dutch white clover plots without banding suggests that this cover crop strongly competes with the trees. We suspect that clover must be a strong competitor with trees for soil moisture (water).
b) Tree height and Basal diameter growth
There were no significant differences (p>0.05) in the tree height growth among the various groundcover management practices. During the second year (2008), trees in plots intercropped with cover crops without banding grew slightly less in height than those in the conventional treatments and plots where a cover crop was intercropped with banding. More years of observations may be needed before any significant difference in the tree height growth can be detected as affected by the various groundcover management practices.
Similar to the tree height growth, no statistical differences (P>0.05) in basal diameter were observed among the various groundcover treatments at the end of the first growing season (2007). However, the second year after the plantation establishment, trees in the conventional plots and the cover crop plots with banding grew bigger than those in plots where a cover crop was intercropped without banding.
Objective #2: Evaluate the effectiveness of the cover crop in weed control
The effectiveness of the cover crops in weed control was assessed by evaluating the cover crops and weed biomass in each treatments. Overall, white clover had the highest annual dry matter production followed by alfalfa and rye grass. For perennial rye grass, the average biomass harvested during the first four clippings (early to mid-season) were higher than the biomass produced from mid to late season. White clover however had better yields in early and late season compared to the mid season harvests. Indeed, the growing season of 2008 was characterized by an extended drought in mid season (July –August) and this may have negatively affected white clover yieds during that period. Alfalfa biomass yields were less affected by the drought which occurred during the middle of the growing season.
The highest weed biomass was recorded in alfalfa plots, yielding 2.48 t ha-1 during the growing season of 2008. There was a trend toward an increase in weed population biomass from early to late season in alfalfa plots. Weed biomass yields, recorded in perennial rye grass and alfalfa plots were similar (1.52 t ha-1) and revealed to be significantly lower than those recorded in alfalfa fields.
Objective #3: Evaluate the effectiveness of integrated systems to chemical leaching
In each treatment suction lysimeters were installed on June 30, 2008 to collect water leaching below the tree rooting zone (1.20 m below the ground). Leachate samples were collected on a weekly basis in each plot from early July through the end of September 2008. In general, there was a slightly higher in conventional plots compared to cover crop plots although differences in July and September were not statistically significant. Slightly higher water in conventional plot can be caused by two factors. First, it could be that cover crops has improved the water holding capacity of the soil, thus reducing the total amount of water moving below the rooting zone. It is also likely that the presence of cover crop increased crop water use in the system therefore, reducing the total volume of water available for leaching.
Leaching of water through the soil profile can occur whenever precipitation exceeds potential evapo-transpiration. The quantity and frequency of leaching is therefore determined by the amount, type and seasonal distribution of precipitation. Vegetation is known to influence leaching losses by decreasing the amount of water and nutrients available to move through the soil profile (Sharpley, 1992). August received less rains (data not shown) than July and September and the difference in the total amount of leached water was statistically significant between alfalfa plots compared to Dutch white clover and the conventional.
The dissolved carbon concentration in water leaching from the rooting-zone was not statistically different among all treatments, although total nitrogen concentrations differed with groundcover management practices. Conventional plots had significantly higher levels of leached nitrogen than plots where cover crops were grown.
The concentrations of N leached in the conventional plots were more than twice those recorded where cover crops were grown. For both alfalfa and Dutch white clover, there was no trend of increased N concentration in leachate with increased nitrogen input. There was no significant difference between Ca2+ in leachate of the conventional treatment and both cover crop species. Potassium concentrations in the conventional plots were twofold higher than those with cover crop.
Although N and K concentrations in leachate were not statistically different between the two cover crops across N fertilization levels, values for alfalfa were slightly lower. Compared to Dutch white clover, alfalfa has a deeper-rooting system than white clover, which may make this cover crop more efficient in recycling nutrients. Indeed, Sharpley et al. (1992) demonstrated that including cover crops in cropping systems or crop rotation increased sustainability with regard to nitrogen recovery when compared to conventional systems. Alfalfa can develop roots to depths greater than 5.5 m has and has been shown to utilize nitrate from any depth where soil solution is extracted by its roots. Mathers et al. (1975) reported that alfalfa removed nitrate from the soil profile at depth of 1.8 m during the first year of establishment and to the depth of 3.6 m during the second and third year of stand development.
In Bolivia, Barber at Navarro (1994) found a significant increase in K in soil from 14 different cover crops plots. This was thought to be an effect of K translocation from the subsoil (Lal et al. 1993) which was also reported by other authors (e.g. Eckert, 1991).
For leached Ca2+, there were no significant differences in concentrations among treatments, possibly due to the fact that Michigan soils, especially the soils in the study area, are known to be calcareous and the availability of this nutrient may far exceed the crop needs. This may also explain the greater concentrations of Ca2+ in leachate samples compared to the other nutrients.
Impacts and Contributions/Outcomes
This paper reported on preliminary research results of the effects of incorporating two leguminous and a non-leguminous cover crop into Fraser fir Christmas tree production systems on soil physical, biological, chemical properties and subsequent tree performances. This report also discussed the influence of groundcover management practices on weed control (through biomass evaluation) and some key nutrient losses through leaching. When managed properly, the inclusion of cover crop can significantly improve soil nitrogen pools, control weed and prevent nutrient leaching. However, inappropriate management could negatively affect the overall tree performances. Future study will focus on the impact of the cover crops on weed population and the tree foliar chemistry. More years of data is needed to evaluate the potential effects of the various groundcover management practices on nutrient cycling and the tree performances. We are also planning to use the field data to develop simulation models that can be used to accurately predict changes in soil nitrogen and carbon pools associated with groundcover management in a Fraser fir plantation.
Liste of publications/ posters
Nikiema P. and P. Nzokou. (2009). Effects of Groundcover Management Practices in an a Fraser fir (Abies Fraseri)-Cover Crop Intercropping System on Soil Microbial Biomass and Community Catabolic Diversity. Paper accepted for oral presentation at 11th North American Agroforestry Conference May 31 – June 3, 200 in Columbia, Missouri
Nikiema P, P. Nzokou, D. Rothstein, B. Cregg. 2008. Effects of Different Groundcover Management in Fraser fir Plantation on Soil pH, Moisture and Microbial Biomass C and N. Proceeding of the 105th Annual Conference of the American Society for Horticultural Science July 21-24, 2008 in Orlando, Florida HortScience. 43 (4): 1184-1184
Nzokou P., P. Nikiema and B. Cregg 2008. “Cover crops and fertility management in Fraser fir production: Avoiding dumping scarce resources down the drain in difficult times. Great Lakes Christmas Tree Journal 2(2): 16-28.
Nikiema P, P. Nzokou, D. et al. (200 ?). Soil Microbial Biomass and Functional Diversity Responses to Intercropping Cover Crops with Fraser Fir (Abies fraseri [Pursh] Poir.) in Michigan, U.S. Manuscript almost finished, yet to be submitted to the Journal “Biology and Fertility of Soils”