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#Polygon
x <- structure(list(AREA = 1489841.35074, geometry = structure(list(
    structure(list(structure(c(4371395.39166986, 4371395.39166986,
    4372634.07252284, 4372634.07252284, 4371395.39166986, 5478810.68590502,
    5480013.45040013, 5480013.45040013, 5478810.68590502, 5478810.68590502
    ), .Dim = c(5L, 2L))), class = c("XY", "POLYGON", "sfg"))), n_empty = 0L, crs = structure(list(
    epsg = NA_integer_, proj4string = "+proj=tmerc +lat_0=0 +lon_0=12 +k=1 +x_0=4500000 +y_0=0 +datum=potsdam +units=m +no_defs"), .Names = c("epsg",
"proj4string"), class = "crs"), class = c("sfc_POLYGON", "sfc"
), precision = 0, bbox = structure(c(4371395.39166986, 5478810.68590502,
4372634.07252284, 5480013.45040013), .Names = c("xmin", "ymin",
"xmax", "ymax"), class = "bbox"))), .Names = c("AREA", "geometry"
), row.names = c(NA, -1L), class = c("sf", "data.frame"), sf_column = "geometry", agr = structure(NA_integer_, class = "factor", .Label = c("constant",
"aggregate", "identity"), .Names = "AREA"))

library(sf)

#distances for buffers                                                                                                                                                                                                                                                                                                                                                                               "aggregate", "identity"), .Names = "AREA"))                                                                                                                                                                                                                                                                                                                        "aggregate", "identity"), .Names = "AREA")
dist <- c(50,500,1000,3000)

#list holding the buffer rings
lstbuffers <-list()
for (i in seq(dist)){
  #first buffer
  if (i == 1){
    lstbuffers[[i]]<- st_buffer(x,dist[i])

  }else{
  #next buffers as rings around the previous
    lstbuffers[[i]] <- st_difference(st_buffer(x,dist[i]),st_buffer(x, dist[i-1]))
  }
  lstbuffers[[i]]$radius <-dist[i] #add the radius
}

buffers <- do.call(rbind,lstbuffers) #DOES NOT WORK, I get a matrix of lists :(