British Columbia Mine Reclamation Symposium

Nitrogen cycling in high elevation reclaimed mine spoil in southeastern British Columbia Fyles, Helen 1984

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th  Proceedings of the 8 Annual British Columbia Mine Reclamation Symposium in Victoria, BC, 1984. The Technical and Research Committee on Reclamation  NITROGEN CYCLING IN HIGH ELEVATION RECLAIMED MINE SPOIL IN SOUTHEASTERN BRITISH COLUMBIA by Helen Fyles Introduction One of the most important nutrients for plant growth, and the nutrient in shortest supply in reclaimed mine spoil, is nitrogen. For these reasons reclaimed areas at Westar must be fertilized annually with 20 to 30 kg N/ha. This fertilization produces sufficient vegetation cover to reduce erosion and, over a period of years, results in a build-up of N in the reclaimed soils. The objectives of all our research on N has been to determine whether or not sufficient N has accumulated in the soil of the older reclaimed sites, to support plant growth without further inputs of fertilizer N. If an adequate reserve of N is present, annual maintenance fertilization can be stopped and the reclaimed grasslands will sustain themselves with no further management. This is one of the final objectives of successful reclamation.  soil/plant system, and see if N was accumulating within the litter compartment. The second objective of the study was to measure fertilizer efficiency: the proportion of the applied fertilizer that was recovered in the plant. Fertilizer is applied each year to the reclaimed areas at considerable expense to the mine. It is important to know if the fertilizer we are using stays in the system and is mostly taken up by plants, or is leached out during heavy rains. If we are losing large amounts, a different kind of fertilizer, such as a slowrelease one, might make fertilization more cost efficient. The third and major part of the study was concerned with estimating the build-up of N in the soil/plant system and assessing to what extent the reclaimed sites were dependent on fertilizer to sustain their productivity. Specifically, the objective was to determine how much of the N in plants growing on reclaimed sites came from the fertilizer, applied in any one year; and how much came from the soil N that had accumulated over the years since site establishment (native N). In the young sites (1-3 years old), it was expected that a large proportion of the N in the growing plant came from fertilizer because soil N levels would be low. In the older sites (4-10 years old) it was expected that much less of the N in the plant was from the fertilizer and a greater proportion was taken up from the soil. We would like to be able to determine the length of time over which annual fertilizer applications are necessary before sufficient N is being cycled within the system to meet plant needs. In this way, management procedures could be terminated after a known period. The latter two objectives of the study were met using a labelled 1 5 N fertilizer.  Background and Objectives Until fairly recently, it was thought that this self-sustaining plant system could not be produced on subalpine sites using agronomic species. The major concern was that fertilization produced high yields of grasses each season and that those grasses seemed to have a high proportion of shoots relative to roots. When the plants died back each fall, a thick layer of litter was formed on the soil surface. Nutrients were continuously being tied up in this dead plant material where they were unavailable to support further plant growth. It was thought that litter decomposition was so slow that it prevented the return of nutrients to the soil and continued fertilization would be necessary to maintain plant growth on the site. This is the basis for the first objective of the study which was to quantify the distribution of N within the different parts of the  83  th  Proceedings of the 8 Annual British Columbia Mine Reclamation Symposium in Victoria, BC, 1984. The Technical and Research Committee on Reclamation  There are two common isotopes of N: normal N has a molecular weight of 14 and the second isotope (5N) has a molecular weight of 15. During analysis of plant or soil samples these two forms of N can be distinguished on the basis of their mass. The fertilizer that was applied to the research plots was enriched with 15N. By sampling at frequent intervals, the presence of the fertilizer could be traced as it moved from the soil surface, into the soil, the microorganisms, the plant roots, crowns and shoots and subsequently into the surface litter (Figure I).  Distribution of Nitrogen in Plant Materials Figure 2 illustrates the distribution of N within the plant materials. The total amount of N in all the plant compartments has been calculated and the proportion of that N in any one compartment is represented by a box. In the two older reclaimed sites and the native grassland, the majority of the plant N (45 to 50%) was in the roots. The youngest site (1980) had much less, about 30% of its plant N, in the root compartment. It is evident that there is a substantial increase in the importance of the roots as the reclaimed site ages. The litter contained considerably less N than the roots in all sites, never having more than one third of the total. Since our selfsustaining grassland had about 30% of its plant N in the litter, we can assume that this is a manageable level on our reclaimed sites. This data suggests that N is accumulating not in the litter but in the roots. It is accummulating where it should in order for a stable system to develop.  The Method  The labelled fertilizer was applied to plots established on four reclaimed sites of different ages in the spring of 1982. 1. The youngest reclaimed site was seeded and fertilized initially in 1980. At the time the labelled fertilizer was applied it was starting its second growing season. 2. The next youngest site selected, was first seeded in the fall of 1977 and was entering its fifth season when the fertilizer was applied. 3. The oldest reclaimed site under study was initially seeded and fertilized in 1974. It was nine years old in the first year of study. 4. A fourth site, an undisturbed native grassland, adjacent to the previous reclaimed site, was included to compare N cycling in the three reclaimed systems with a natural system. The grassland had a southwest exposure and was much warmer than any of the reclaimed sites. It tended to start growing earlier in the spring and die back earlier in the fall. Three sets of samples were taken from these sites during the 1982 growing season in June, August, and September. Two more samplings were carried out in June and August of the following year. The entire system was sampled: the soil and roots to a depth of IO cm, the surface litter, the shoots and the crowns. The crown is that part of the plant from which the roots and shoots originate and is considered to be a storage organ and therefore important in the ability of the plant to overwinter.  Decomposition  It is evident that N is not accumulating in the litter compartment therefore significant amounts of litter must be decomposing each year and returning nutrients to the soil. It is difficult to calculate the amount of litter decomposed annually because additions and losses occur continuously over the growing season. However, it is possible to measure the losses which occurred between late fall and early spring. The shoots present in September 1982 entered the litter pool as dieback occurred and winter began. The total amount of litter present in the late fall can therefore be calculated (Table 1). The amount of litter remaining on the soil surface the following spring was measured and the difference between the two values was the amount decomposed during the winter. In the three reclaimed sites, litter loss amounted to 51%, 41%, and 35% of the litter that was there in the fall. Several studies carried out in the United States, have also measured substantial litter losses over winter  84  th  Proceedings of the 8 Annual British Columbia Mine Reclamation Symposium in Victoria, BC, 1984. The Technical and Research Committee on Reclamation  Figure I Movement of Fertilizer Nitrogen  85  th  Proceedings of the 8 Annual British Columbia Mine Reclamation Symposium in Victoria, BC, 1984. The Technical and Research Committee on Reclamation  Figure 2 Distribution Of N Within the Plant Component August 1983  86  th  Proceedings of the 8 Annual British Columbia Mine Reclamation Symposium in Victoria, BC, 1984. The Technical and Research Committee on Reclamation  Table I Litter Decomposition Between September 1982 and June 1983  87  th  Proceedings of the 8 Annual British Columbia Mine Reclamation Symposium in Victoria, BC, 1984. The Technical and Research Committee on Reclamation  (Bleak 1970, Knight and Kyte 1975, McBrayer and Cromack I960. Apparently, once sufficient snowpack has accumulated to buffer the soil surface from extremes of air temperature, the temperature of the litter zone rises above 0° C, and decomposition is initiated. Litter decomposition also occurred during the summer months. However, it was more difficult to measure because plant material was continually dying, sloughing off and being added to the litter pool. The net loss of litter during the summer of 1982 was measured by subtracting the amount present at the end of the growing season from what was there at the beginning. All sites showed a net loss in litter (Table 2). In the grassland, the amount of litter actually increased over the summer. As mentioned earlier, this site died back earlier in the season and by September 1982, many of the plants had already died and entered the litter compartment, accounting for the increase. If the net loss of litter during the summer was added to the amount decomposed over winter, litter decomposition over the entire year could be estimated. In order to determine whether litter was accumulating on the reclaimed sites, the amount of litter added during the year was compared to the annual amount decomposed. Total additions to the litter pool during the growing season were approximately equivalent to peak shoot growth in 1982. A comparison of estimated litter additions with estimated losses over the entire year revealed that only in the youngest sites was litter accumulating. All other sites showed a net loss of litter. It is unlikely that these sites were actually losing more litter than was added, over the long term. Either this was an unusually good year for decomposition or the previous year had exceptionally high shoot production. It is likely that these sites are "breaking even"; as much litter decomposes as is added each year and, in general, the size of the litter compartment remains constant. The amount of N recycled by this litter can be substantial. In the oldest reclaimed sites, the quantity of N returned to the soil  via decomposition processes was approximately 30 kg/ha. This is as much N as is present in the shoots at peak growth and is a similar amount to what is applied annually as fertilizer.  Fertilization Efficiency An increase in fertilizer efficiency is evident with site age in August 1982 (Table 3). The young reclaimed site recovered about 30% of the applied fertilizer in its plants and the two older reclaimed sites 70 to 75%. The grassland recovered 50% of the fertilizer but statistics do not distinguish any significant difference between the three older sites. The efficiency with which plants on the older sites use the fertilizer N is probably due to their large root system providing a greater capacity for uptake. Plants on the 1980 site with much smaller rooting systems, take up much less N. In August I 983, over one year after the fertilizer was applied, there was still a high proportion of fertilizer recovered in the plants. This suggests that the fertilizer applied in any one year can be an important source of nutrients the following year. Recovery of Fertilizer in the Soil Plant System It was apparent that only a portion of the fertilizer was recovered in the plant and it was important to determine the fate of the rest of the fertilizer. The proportion of fertilizer recovered in the entire soil/plant system, 1.5 years after its application, is recorded in Table 4. In the reclaimed sites, about 20% of the fertilizer was lost and in the grassland site apparently none was lost. All these values are very high and suggest that the fertilizer is staying in the soil or plant material for at least a couple of years after application. Note that even in the 1980 site where only 30% of the fertilizer was recovered in the plants, very little was lost from the system. Even though it apparently remained in the soil for a year, the fertilizer did not leach out.  88  th  Proceedings of the 8 Annual British Columbia Mine Reclamation Symposium in Victoria, BC, 1984. The Technical and Research Committee on Reclamation  Table 2 Annual Litter Decomposition  89  th  Proceedings of the 8 Annual British Columbia Mine Reclamation Symposium in Victoria, BC, 1984. The Technical and Research Committee on Reclamation  Table 3 Proportion of Fertilizer Nitrogen Recovered in the Living Plant (%)  SITE  AUGUST 1982  AUGUST 1983  1980  31  23  1977  69  39  1974  76  35  GR.  51  31  90  th  Proceedings of the 8 Annual British Columbia Mine Reclamation Symposium in Victoria, BC, 1984. The Technical and Research Committee on Reclamation  Table 4 Fertilizer Recovered 1.5 Years After Application  SITE  FERTILIZER RECOVERED(Vo)  1980  82  1977  81  1974  77  GR  103  91  th  Proceedings of the 8 Annual British Columbia Mine Reclamation Symposium in Victoria, BC, 1984. The Technical and Research Committee on Reclamation  Proportion of Plant Nitrogen Derived from Fertilizer Figures 3 and 4 show the proportion of N in the living plant that is derived from the fertilizer. At peak standing crop in 1982, on the youngest reclaimed site, almost 40% of the N in the growing plant came from the fertilizer. In the 1977 site, about 20% of the N came from the fertilizer and in the 1974 site about 10%. Plants growing in the native grassland derived about 6% of their N from the fertilizer. There is a tremendous decline in the dependence of the plant on fertilizer for its N, 2 to 5 years after initial establishment. By the time a site is five years old, its plants are deriving 80% of their required N from the native N in the soil and only 20% from the fertilizer. This means that a substantial pool of N has built up in the older reclaimed systems and that N cycling is continuously resupplying the plants with necessary nutrients. Examination of similar data for the following spring (June 1983) show that these differences are even more distinct. In the youngest reclaimed site, a high proportion of the N in the plants is from the fertilizer, while in the other sites only IO to 14% of the N is from the fertilizer. These data suggest that fertilized reclaimed sites build up a substantial pool of soil nitrogen within five years. After five years, the benefits of fertilizer decline as the sites become increasingly self-sufficient.  but substantial amounts are being recycled by decomposition processes. A large proportion of the plant N is present in the plant roots; where it should be for a stable, self-sustaining plant system to develop. The labelled fertilizer study has shown that in the older reclaimed sites, only a small portion of the N in the plant comes from the fertilizer and most of it comes from supplies accumulated in the soil. This suggests that present management practices at Westar are successful I Iy creating plant/soil systems on their high elevation reclaimed sites which recycle sufficient nutrients to meet the needs of plant growth.  Acknowledgements  The joint support of Westar Mining Ltd. and the University of Alberta was instrumental in this research. Also, I gratefully acknowledge the financial contribution to this research by Alberta Environment.  References  Bleak, A.T. 1970. Disappearance of plant material under a winter snow cover. Ecology 51:915-917. Knight, D.H. and C.R. Kyte. 1975. The effect of snow accumulation on litter decomposition and nutrient leaching. In: Medicine Bow Ecology Project. The potential sensitivity of various ecosystems. USDI Bur. Reel.  Summary  The research that has been carried out on N at Westar has been essential to determine the ability of the reclaimed sites to survive once management practices are terminated. It has been established that N is not accumulating in the litter on the soil surface  McBrayer, J.F. and K. Cromack. 1980. Effect of snow-pack on oak litter breakdown and nutrient release in a Minnesota forest. Pedobiologia 20:47-54.  92  th  Proceedings of the 8 Annual British Columbia Mine Reclamation Symposium in Victoria, BC, 1984. The Technical and Research Committee on Reclamation  Figure 3 Proportion of N in the Living Plant Derived From Fertilizer September 1982  93  th  Proceedings of the 8 Annual British Columbia Mine Reclamation Symposium in Victoria, BC, 1984. The Technical and Research Committee on Reclamation  Figure 4 Proportion of N in the Living Plant Derived From Fertilizer Spring 1983  94  

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